My point has already been made. Any Soviet victory ever is treated as ASB, because we all know they were nothing but Slavic hordes of cannon fodder; and that you're asking us to assume Stalin is an idiot.
Not ASB, no. Just not the most expected result. The Soviets may indeed win conventional WWII for good (but they still have to fear American nukes) IF they can overrun Germany, Italy, and France in the first Blitzkrieg strategic offensive. This is quite possible in the 1942-43 scenario, but not the most likely outcome, since a Land-Lease-less Red Army would be much less mobile, and they would be rather overextended just going from their 1939 borders to Stettin and Trieste in one continous move. OTOH, if they are stopped at that point, the substantial superior Euro manpower/industrial potential would soon come into play. Basically, this war would play much like a reverse Barbarossa: if the attacker can't overrun the enemy's vital centers in the first strategic offensive, time runs a lot against them.
Nobody is treating the Soviets like dumb cannon fodder, here: notice how they are commonly assumed to overrun Scandinavia, Poland, Eastern Germany, Slovakia, Hungary, the Balkans, and the Middle East, before getting stalemated and pushed back. That's a quite remarkable accomplishment, just like OTL conquests of Nazi Germany. However, they are facing a stronger coalition at the start than Hitler, and America is still rather likely to join the anti-totalitarian crusade in the end. OTOH, just because they won IOTL by eventually achieving decent competence (but being war gods by no means), tapping and exhausting all their resources, and getting a huge help by America, we do not need to deem reasonable that they would walz to the Channel without a sweat.
As it concerns Stalin, he was no idiot, but he was rather more paranoid than Hitler, only somewhat less megalomanic and a gambler, but just as prone to make bad mistakes about his enemies' ability or willingness to fight (Winter War, Barbarossa, Korean War, Titoist split). As such, in a TL where he got no sobering Barbarossa experience, it is wholly plausible that he would underestimate the ability or willingness to fight of the Western powers, and hence think he come get away with snatching Tsarist claims, until he makes one aggression too many, or deem that the capitalist powers are making a united front against him (and he has no german-western divide to exploit), and hence decided he needs to unleash a pre-emptive attack.
From our hindsight PoV, the Soviet decision to invade Europe only looks foolhardy because we take respective industrial potentials, strategic and logistic overstretch, and American intervention into consideration. However, such issues were not usually taken much into account by WWII leaders, especially not by the dictators. Therefore, with a fully expanded and modernized Red Army, Stalin may easily feel he has a reasonably good chance of overwhelming the capitalist powers before they can muster. Most likely a wrong gamble, in light of the above factors, but not a dumb one.
I did make a constructive post. Here's another. The Nazis were also hammered by the Finns, in 1944.
I fail to see how this would be relevant to our scenario at all.
Here's another: Soviet intelligence and counterintelligence was the best in the world.
In our WWII, they were helped considerably by the fact that they were official allies during most of the war, so the western powers lowered the guard against Soviet espionage and Communist infiltration. They would not have such a free ride in a WWII where they are the hated enemy of western civilization. Intelligence-wise, they would probably fare somewhat better than OTL Nazis, but not substantially more so. Most Communist sympathizers, a huge help to WWII Soviet espionage IOTL, would be singled out and interned wthin days or weeks of the declaration of war. Western intelligence services and patriotic citizens would be on the lookout for people that expressed Communist sympathies in the past. The likes of Kim Philby and Klaus Fuchs would be rooted out.
A third: Hell would freeze over before the Poles give up their coastline.
ITTL, Britain and France are all but guaranteed to look the other way when Germany claims the Corridor, either before or after the war. And without them, Poland has no chance of resisting. They would either be forced to back down in Munich II conference, crushed into a quick German-Polish war before the Soviet attack, or the German armies, who played a decisive part into kicking back the Soviet hordes, would claim back the devastated area when the Red Army is pushed back. Britain and France would give as much scarce concern to Polish post-war claims on the Corridor as they did on their laims on eastern territories IOTL.
Besides, after the war, Poland, like the rest of Eastern Europe, would have bigger, more basic concerns than Danzig or the Corridor. After several years of being a major battlefield, and a merry atrocity playground for the NKVD, Poland would be in such a devastated state that they would be in dire need of German and Euro help for survival and reconstruction, and probably at least somewhat grateful for being liberated. They would get abudant economic help in exchange for forgetting their ill-conceived 1919 claims. Morevoer, ITTL they would keep all their pre-war eastern territories, and maybe add some bit, so they fare not so bad at all.