What if it just stopped at step 5, except maybe all of France goes Communist and more or less Stalinist. Perhaps, as in Yugoslavia, French Communists have enough independent spirit and political traction that they throw off Stalin's rule at some point but remain in charge on their own terms. A Red-Army liberated France is of course a very different situation than Tito's power in Yugoslavia--but I imagine that as the Third Reich collapses in the East, being plowed under by the advancing Red Army machine, a more or less Stalinist controlled uprising breaks out in France, when the Germans are too weak to turn around and crush it. So although lacking the long track record, street cred and half-decade long loyalty of his followers Tito had and having little time to accumulate and test the sheer numbers of Party fighters as a proportion of population Tito had, still the Red Army and NKVD or whatever initials it is using that decade find the French touchy and harder to control than in Nazi-steamrollered Eastern Europe.
Or not; the numbers of Soviet occupiers might simply be high enough, or the strength of the French Party's grassroots connections too weak, and France remains under Moscow's control.
Now I see that you do have D-Day in 1945, which would tend to liberate part of France on Western terms--and indeed in a scenario where the Russians have more punch and the Western Allies are delayed a year, the Red Army is probably right about on the point of breaking through the last bits of Germany in the west, when the first US/UK/Free French troops hit the beaches. The Soviets will feel about D-Day the way we feel about the USSR's declaration of war on Japan and invasion--after the Hiroshima and Nagasaki bombs went off. Certainly if Western and Soviet troops meet in the middle of France instead of in Germany, everything then hinges on politics--will the Soviets meekly surrender eastern France to De Gaulle? Will De Gaulle and his Anglo-American patrons allow the Soviets to hang on to it, along with all of Italy and Germany? Will Stalin propose some thing like withdrawal of the Red Army to Germany (but not all his secret agents, only the ostensible ones) and then count on a combination of gratitude and political skulduggery to put in an elected Communist government which promptly and irrevocably allies with him? (In that case a future rupture might be more possible, but I'd predict that in Stalin's lifetime does not happen for the same reasons Mao let it appear that he was under Stalin's thumb even while kicking all sorts of sand in his eyes within the bloc).
Or as you say, France is partitioned.
I don't see step 6 as particularly coupled to the worse situation on the ground though. I think that if the breaks were such that the W-Allies were delayed and the Reds advanced, on paper the same cordiality and collegiality would prevail as OTL--with the same naysayers and Cassandras, followed by the same vigorous reaction of the rump West, which after all was mainly muscled by potential US power. In other words, of course Churchill would be pulling his hair out at the notion of this sweeping Red tide, but while Hitler remains to be defeated will continue to treat with Stalin (even as he has reservations and some contingency plans cooking) and FDR will continue to more or less trust him.
Perhaps in this objectively more dire situation the USA does not demobilize to the near total extent it did OTL. Perhaps Harry Truman cannot win election in '48--though frankly I would not bet on that; it could be that by then he will have had ample opportunity to show his mettle and that combined with his scrappy populism may make his victory less of a squeaker, though perhaps as much of a surprise to the establishment. Or hell, Dewey or whoever wins.
I just don't think the USA and Britain are so ready to go to war with Stalin, regardless of how much territory he holds--indeed the more he holds, the more unwise just shooting from the hip will seem. The USA after all would presumably still have the A-bomb monopoly, for what little that was worth. I suppose if Stalin gets hold of part of France he will be able to take Belgium and the Netherlands too, and then there is nothing to protect Denmark at all--but I don't think he'll invade Sweden and any forces that go to Norway overland will be tiny and not able to take control. So the two big peninsular Scandinavian countries are out of Stalin's hands, as is Spain and Portugal.
In this case I don't think Stalin can afford to move on the Middle East, not by means of invasion--promoting strong revolutionary movements and taking his chances the Anglo-Americans would be desperate enough to construe that as an act of war maybe, but even that scared him OTL.
Ours is a TL where the west controlled the low countries, Italy, all of France, and the lion's share of Germany while Denmark too stood on our side; it may be that Eurocentric Bolsheviks will not see the USA as being essentially enough of the Capitalist nations to count. But if the USA does not demobilize--if we set as a condition of that Soviet withdrawal to a frontier within a Germany we never struck a direct blow against on the ground, and Stalin decides to defy that--I think the level of force we alone could maintain, in combination with the growing threat of nuclear bombardment, would indeed check him. Britain too although we know from OTL was on the ropes economically does have a war economy that involves no starvation (in Britain, India is another story and a weak point) and can produce serious amounts of war material, and a Commonwealth fully mobilized and trained. To an extent British commitment will suffer a bit from sympathy with the Russians, but that will evaporate fast if they do stuff like try to control all of France clearly against French democratic wishes, and will treat all of Germany and the huge bulk of the European continent besides as spoils of war. Which one supposes Stalin will do.
Could even Indian independence be off the table? Well, Labour was committed to it, and OTL the War Cabinet committed long before V-E day to holding elections and setting up a regular Ministry after V-E day, regardless of the status of the Pacific Theatre. Perhaps ITTL the WC will postpone and Churchill stays in charge at least until after V-J day. And perhaps the Tories are more popular due to the dire Soviet threat? But again, perhaps Labour, which included plenty of patriots skeptical of Bolshevism, will take a harder line. Regarding India, I suppose a Labour government, even facing the Reds so closely across the seas, will keep the commitment to turn India loose-but Churchill might not! Any failure of the British to honor Indian nationalism will mean trouble, but perhaps in the circumstances Churchill can overcome his bigotry enough to make an appeal to Indian conservatives alarmed by the Soviets right north of them, or conceivably these conservatives can band together and appeal even to a Labour government that perhaps some stronger form of commonwealth with the Empire is on second thought a very good idea.
Britain is going to be pretty down and out after the war, dependent on Yankee largesse--but that largesse may be forthcoming, for pragmatic reasons iced with sentimental ones, given the severe crisis in Europe and possibly globally, especially as the Maoist Reds in China will be in the ascendency. If the USA remains on a partial war footing, demobilizing some but not all troops--say, half of them--and the war material plants find their orders are cut back but not to nothing, with some being repurposed to civil production but others rolling along, with US rationing relaxed but not eliminated and price controls and semi-voluntary wartime industrial planning still in place, some might predict a less prosperous USA and in certain senses this will be clearly the case--the massive consumer boom of the later postwar period will be delayed, denatured or never happen. But to an extent, even under rationing and price controls and to an extent as much because of central planning as despite it, the US economy, assuming no immediate major war, will surge upward anyway. In wartime conditions poor Americas ate better than they had in decades; housing was a serious problem but might be better solved by a New Deal heritage US government with war powers than the laissez-faire solutions of OTL. Job security will be very high, inflation manageable (or let run wild but wages will spiral up behind it pretty fast with schemes to protect the real value of most capital holdings). One question might be how will US organized labor react to Soviet near-hegemony? Some leaders will regard Stalin as more or less on their side, but I think by and large US labor will be more skeptical than conservative Labourites in Britain will be. They will react badly to what they regard as open betrayals by Truman or whoever, but it is possible a New Deal heritage government still mobilized for possible war, even against a Red enemy, will instead cut ad hoc deals with labor, getting them to extend their wartime no-strike pledges for concrete gains and security. Perhaps the scope of organization will be hemmed in but the already unionized will have solid assurances of their safety. Or maybe the regime allows the unions to organize to their heart's content, assuring the capitalists that even with total organization the power of the unions will be kept within limits they can live with. Taftites might die of apoplexy but more moderate Republicans can accept it. (For that matter Truman might have more trouble from conservative Southern Democrats than the Taftites!)
Under these conditions, Truman (I assume he runs the crucial first few years unless we suppose the ATL war situation would butterfly his nomination by FDR in favor of someone assumed to have more gravitas--but who would that be? Not Wallace for sure! Regardless of any feelings I may have about Wallace, in a TL with Stalin bidding fair to conquer all of Europe in 1944, there is no way the party bigwigs would let Wallace keep the VP position. So I think Truman is pretty likely to inherit the Presidency--unless one supposes the Republicans could win in '44, but I don't think that would happen either). OK Truman will retain substantial military force, much of it deployed in France already. Britain would be no easy pushover, the RN at maximum levels of effectiveness as is the RAF; Stalin has no navy to speak of. That can change in a decade or so but in the short run, the Reds are less of a military threat to Britain than Hitler was. The Russians do have a lot of submarines and have presumably captured the U-boat works, but the RN and USN are pretty good at ASW by now. The massive Red Army, enriched over OTL by the pickings of Italy, all of Germany, and the Low Countries atop their OTL spoils, certainly might threaten to crush the combined Anglo/American/Free French forces like bugs--but even given overwhelming numbers that will eventually prevail, the W-Allied forces are salted and well-armed and won't go down without a nasty fight. The Red Army is spread thinner than OTL, offset maybe by greater numbers due to the early good fortune mentioned, but surely that margin is worn down again somewhat conquering all of Italy, West Germany, the low countries and east France. They are unwelcome occupiers to large numbers of Europeans wherever they are, with only limited help from local political allies--who are themselves pretty fearful and possibly willing to cut a deal with the Yankees and friends. On paper, Stalin can maybe take the W-Allies in France. He knows that what they will do in fact is give a good account of themselves, then retreat again to Britain where he can't touch them. And just maybe the W-Allied forces already in France, knowing that the entire might of the USA and Commonwealth are backing them, benefiting from heavy air support that can probably achieve air superiority over the Red air forces, will hunker down instead, and reinforcements from semi-demobilized Britain will quickly be raised and poured in, followed by legions of Yankees, Canadians and Australians armed with the best weapons on Earth. Oh, and Truman maybe only has a few A-bombs, and maybe Red air defenses can knock out their bombers--and maybe not, and suddenly there's a huge radioactive hole in Stalin's front lines or a key logistic center is gone. All the W-Allies have to do really is hold the line while the USAF gets better at penetrating through and one by one, key industrial and logistic centers get blown away. If the Soviet soldiers were fighting to secure their Motherland, he might be able to rely on them, but they are far from home, embedded in a bunch of countries whose surviving peoples generally don't like them much, with the W-Allies perhaps dropping weapons and ammo for partisans. It could get very very ugly.
The Westerners, despite the desperate urging of the Free French keen to see the rest of France liberated, can afford to stand pat, if they stay mobilized. The US economy is not hurting for lack of the American soldiers, it is running at full steam and more productive than ever. Britain, with a sympathetic USA backing them, can contribute a good share too. The USA is probably putting any anti colonial rhetoric on the back burner. Or simply taking over the colonial role de facto, in the guise of assisting fledgling new republics--basically in the places the Japanese occupied I don't think the Americans will turn them over to rump France but instead set them up as new allied nations. Britain can hold all the colonies she could take back herself, and France keeps the colonies that never changed hands. It is not necessary to demobilize! Given the stark situation I think most Tommies and GI's would accept that the duration is not over yet, and that they'll go home only on rotation--if the war does not flare out they will be mustered out in a few more years, replaced by new recruits, but a huge army is here to stay.
Given the flashpoints of OTL history one might guess WWIII is inevitable, but given the balance of terror of OTL and seeing it as I do as being partially a matter of two super-blocks led by people who didn't actually want a war if they could help it, I think the big war is as avoidable as OTL. To reiterate a theory mine, Stalin is the Great Procrastinator. Ideology and his personal instinct of tyranny both urge him to aggressive war, but he knows he is a sucky general, whereas anyone competent at it he entrusts to run the war for him becomes a potential rival to remove and replace him. The ideal situation for him is the just completed Great Patriotic War, but that is done with, Hitler is gone, the Japanese warlords are no more, and now he must turn his army on the allies who have been helping them. He would lose no sleep over that but I think that his political calculation is, the Soviet ruled peoples must know that it was the greedy capitalists who struck first. And on the other side of the line, the capitalists may indeed be greedy, but also war-weary. Although Stalin may rule Europe, the Anglo-Americans control the globe that made Europe rich, and between them have plenty of industry to fill the role of the entire First World, and moral capital enough to make controlling the Third World seem a solvable problem. Objectively they face a worse problem than we did OTL, but the Soviets are super powerful mainly in potential. They have a vast army in hand to be sure, but that army is likely to be stuck merely as a dead weight on the regime, kept mobilized for reasons of preparedness and defense. Can the Soviets now develop the air and sea power to threaten the USA? Or even Britain? If they can it will take time, time in which the western powers too can build up their capabilities even further and still live comfortable lives while doing so.
I think they just sit and glare as OTL. Western France is a permanent bastion and threat to the bloc, a tripwire for global nuclear war. Oceania calls the shots outside of Soviet Eurasia--and we know from OTL that EastAsia will hive off soon enough. Under the necessarily harsh and inefficient Soviet management, the resources of Western Europe will not change the balance of power so much; Ivan will remain objectively behind.
I suppose the Soviets will build up a big navy, but it will not be too hard for combined USN/RN force to hold in check. Eventually there will be missiles, until then the Soviets are at the mercy of the USAF protected mainly by their large expanses of territory and finite US resources.
I do think given your initial conditions Stalin can incorporate Turkey, Greece, Italy and parts of France, though perhaps not any great ports in the latter. He might get away with trying to take the French mandated territory of Lebanon and Syria but to press on down into Palestine and beyond would be to declare war with Britain and hence the USA, something that won't happen until Hitler is vanquished. And then, not then either.
It doesn't matter than the USSR probably can win on many fronts. The point is, starting a war is starting a war, a war with Great Britain and the USA that will not end until one side or the other is defeated. There would be little more reason for Stalin to think that the war will be won by the Soviet side in this ATL than in OTL. The core of the Soviet Union is devastated and a major reason the Red Army does not go home but remains deployed is that there are few homes standing for the soldiers to live in should they return to where they came from in Russia. Britain is bomb-damaged but they repaired around that, otherwise fully operational as is the USA, and the USA has the ability to make A-bombs, something that cannot destroy the USSR today, but can within a half decade or so. Now would indeed be the time to knock the two opposing Great Powers out but the one thing the great Red Army lacks is global reach, even Britain can defend against everything he's got.
I suppose one factor I usually assume prevents Americans from starting World War Three does not apply here...Stalin is not holding Europe hostage. Stalin has already conquered Europe and only later will be holding small remnants of it--perhaps Scandinavia, and surely the British Isles and Iberia---hostage with threats of destruction, otherwise the only way to free Europe is to defeat Stalin, or hope he chokes on an empire bigger than he can swallow. The W-Allies, leaving out the French anyway, have nothing to lose. So perhaps for that reason the importunities of the hawks who argue the war is inevitable, has really in a clear sense already started, and should be ended by the western powers at whatever cost now because it will only cost more to do so later-indeed, later the Reds might win, will prevail after all.
But I don't see Truman, or Eisenhower, being the first one to strike, nor will any British leader persuade either that it must be done by a Western first strike. Some other combination of leadership might do it perhaps, particularly with Churchill in the mix, but if the West does not strike first, I don't see Stalin doing it either, not when he has practically all of Europe to play around with and the rest of the world to seek to win over through politics.