is the lack of L-L crucial or do the planes & tanks churned out by the factories east of the Urals still overwhelm the Germans?
Likewise, on the question of Moscow first or deal with the Kiev 'pocket' - any ATL with Moscow first?
IMHO, as I understand it, once the Germans had lost at Stalingrad, that was it. They were on the back foot and retreating until the bitter end after that.
Meanwhile, Lend-Lease to the Soviet Union was indeed massive, but it took time to build up. Looking at tonnages or any other numerical index of value of goods shipped there, it rose pretty much linearly between the weeks after Barbarossa was launched (USA sent nothing to USSR directly until we entered the war, but Churchill diverted much of the goods being sent to UK along with some British products almost immediately) until some months before V-E Day when the USA abruptly pulled the plug. Quarter after quarter, the goods shipped to (arriving at I mean...quite a lot was lost to German raiding along the way) the Soviet Union increased steadily.
Therefore some elementary calculus shows us that the cumulative mass of goods arriving in the Soviet Union went as the square of elapsed time. Therefore the cumulative total shipped to the Soviet Union as of the Soviet victory at Stalingrad was only a fraction of the total, a small fraction, less than a quarter by far, of the total mass shipped.
I conclude that while this kit was useful already to the Russians (bearing in mind that it also took time for the Americans to get on the same page with the Soviets as to what kind of stuff to send, initially we were shipping stuff not particularly necessary or useful to them, so that discounts the importance of early LL as well) it probably was not crucial to their survival. Taking terrible damage and losing vast swathes of their most valuable territory and people to German control, they nevertheless retained enough force and productive assets to stop the advance and start pushing it back pretty much all by themselves.
I conclude further then that had the Western Allies given the Soviets zero support of any kind, ignoring their efforts completely, the Soviets would still have stopped and then begun to reverse German advances before losing too much of their own assets, and then after that the Reich was doomed. Fundamentally, it was a fatal mistake for Hitler to attack the Soviets at all. Conceivably if he had restricted himself to limited advances, say taking all of Poland's eastern territories and chosen a strategic line somewhat beyond the prewar Soviet border, then dug in there, or taken just Ukraine and the Caucasian oil fields perhaps, and dug in there, with a suitably conciliatory policy encouraging the conquered peoples to positively support the Reich hegemony, it might have worked OK, but then of course he'd have a long bitterly contested border with the Soviet remnant. Establishing a track record of "better ruled by Germans than Stalin's Bolsheviks" might have undermined Soviet morale enough to make this sort of feasible, but the cost/benefit of trying to hold that border against remaining Soviet power would be quite dubious despite the rich resources a southern strategy would win him. And of course being conciliatory would go directly against the grain of Nazi notions; a cynical enough alternate version of Hitler might feasibly order it but it strikes me as hideously unstable. Given Hitler's true character, and that of many of his Nazi acolytes and general German arrogance, practically ASB. A different German regime might do it but I suspect such a regime would be deterred by a more rational cost/benefit calculus from attempting such grandiose schemes despite near success in the Great War.
No matter how we slice it, conquering Russia is a dubious prospect at best.
Now I have seen it argued that once LL shook down to be pretty much what Russians were ordering and not a random grab bag of what Americans quarreling over whether we should be doing this at all and grudgingly giving them leftovers from American priorities as was the case early on, much of the tonnage was trucks. Generically speaking, then, motor vehicles that were not heavily armored nor intended for front line combat, but vital toward giving the Red Army the rapid mobility that enabled them to keep the pressure on the various Axis forces (mainly Wehrmacht and Waffen SS, but the latter involved lots of generic European recruits, and alongside were brigades from various subjected European puppet regimes and Italy) and advance to Berlin and other points far west. It is argued that without these crucial supplies, the Soviets would have perforce had to either do without motor mobility almost completely or to achieve a more balanced force, cut drastically down on what their industry did concentrate on--so they'd have fewer tanks, artillery, rocket batteries, planes, and other OTL Soviet made munitions and kit to enable the trucks to be made.
Slowing Soviet advances in this way could, I suppose, be argued to give the Reich sufficient respite to match Soviet production and slow their westward advance quite a lot, and given the deep cynicism of both dictators I suppose a separate truce with the Reich continuing to hold large swathes of former Soviet territory might not be ruled out.
It hardly seems like a slam dunk to me though.
Overall, the Soviets accounted for about 2/3 of all Euro-Axis losses. Even if we assume the British threw in the towel and the USA never engaged the European Axis at all, focusing exclusively on Japan or sitting out the war completely (surely if Britain had a truce with Hitler, nothing would stop them from reinforcing their holdings in the Pacific theatre and this would make the Japanese project of late 1941 look very dubious indeed--they might feel they would do better to pile on to the general assault on the Soviet Union perhaps despite the plastering Zhukhov gave them in their brief skirmish some years before--therefore no plans to take the Philippines, no invasion of SE Asia generally, no Pearl Harbor) I think the USSR could, especially if the war were drawn out several more years, eventually muster 50 percent more capability to be expended. Taking more time implies lower intensity and fewer losses in any given period, so it is not right to simply extrapolate OTL loss rates and calculate Soviet exhaustion from that. Time bought wins both sides extra strength. A truce with Britain would open the sea lanes to Reich hegemonic territory to import vital goods. But the question is, would the Reich be able to purchase such goods? They have the French colonial territories under Vichy...perhaps. It is a question of what terms it would take to get Britain to bow out and cease violent combat. Given British balance of power conventional wisdom I can see the British driving a hard bargain in which Vichy France must relinquish colonial control, or perhaps a tacit understanding that they can claim control on paper all they like but British efforts to subvert that control and switch administration over to a "Free French" authority would not be regarded as a deal breaker.
Such concessions by the Western powers to Reich legitimacy might make the Soviet struggle that much more stark and brutal. But it is not clear to me that they were nearly so close to breaking as many Monday morning quarterbacks assume. I think their collective desire to resist German conquest was pretty tenacious and that their control of resources in the more eastern reaches of their territory was sufficient to give them the means of driving the Germans out. And once the steamroller got rolling, it would keep rolling, prewar borders be damned, and so even Britain opting out and thus preempting the USA from ever being involved at all would merely postpone the fall of Berlin, not prevent it. And the Reds would just keep rolling west with nothing to stop them short of the Atlantic. It might take them into the 1950s to finish the job, but certainly if Hitler could not break them, they aren't going to be stopped by anything short of the Western powers allying
with Hitler. If even that would work.
So no, I don't think denial of LL would tip the balance. And any gains Hitler got temporarily by delaying the Soviet recoil would, if the war in the west continues, be diverted to more ability to fight on that front, meaning higher losses for the Anglo-American led struggle there.
Given the co-belligerence of both Western Allies and the Soviets, formal inclusion of all into a Grand Alliance and cooperation in the form of Lend Lease aid to the Soviets was far and away the most rational policy. Americans had productivity out of proportion to our ability to field fighting men on the ground (we could have had a bigger army had we lavished less on air power to be sure, nor did our domestic mobilization come close to maximum capacity either) and the Russians had personnel (men and women) who were starved for kit, so putting US made kit in Soviet hands was quite efficient, and saved western lives on a considerable scale. A policy of cold separate co-belligerence with no coordination would be pretty stupid, as Churchill saw plainly despite his visceral anti-Communism, nor would FDR be inclined to just let the Soviets twist in the wind. A different American President might have dictated a different result, but the kinds of Republicans most likely to win elections in the Depression era USA tended to be New Dealers Lite and probably would at least reluctantly listen to the wisdom of Lend Lease. Perhaps an untimely death for FDR might have placed some Democrat more inclined to hard-shell conservatism such as the two-term VP Garner of Texas into office--but I daresay Garner would either have let himself be ushered to lead the New Deal parade if he knew what was good for his electoral prospects or bow out or be shoved out of the way if he were too stubborn to listen to reason.
Lend Lease was the smart thing to do, and asking for it not to happen is demanding Western leadership be handed an Idiot Ball IMHO.