It isn't that hard to imagine Stalin instantly betraying the Western Allies
Yeah it is. It's worth considering this exchange between Stalin and an Allied official at Yalta, discussing Hitler.
Stalin: Hitler was a very clever man, but he had one flaw... he didn't know when to stop.
American: Does anyone ever know when to stop?
Stalin: I do.
Stalin knew that the USSR against the west would be able to win battles and campaigns, but it wasn't able to yet win
wars against them. So he very much wasn't interested in a war with the capitalist west yet.
There was a reason why the Russian troops liberated from POW camps in 1945 were fed for a few days and then given a rifle and pointed west. The Russians were running out of manpower.
Myth. Soviet practice of impressment of freshly liberated back prisoners date back to the first year of the war and had little to do with their manpower situation. The Soviets in '45 were facing some hard choices in manpower, but they could sustain the strength of the Red Army in the face of the losses they were taking. They certainly weren't down to the level of the British, who were having to disband divisions wholesale.
The Western Allies have around 100 division-equivalents in Western Europe. The Soviets had just shy of 500 division-equivalents... although if we want to compute for the different strengths in stuff like manpower and equipment, the number is around ~250 Western division equivalents (the planners for Unthinkable came up with 264, which is roughly in the same ballpark). Regardless, the WAllies are outweighed in combat power on the order of approximately 2.5:1. The air war, at least in the short term, is a wash. Air power needs breathing space to affect the ground war, something that won't exist here. The naval war is obviously the WAllies from the outset, but is otherwise irrelevant to actual combat operations.
In any case, the most likely way this plays out is the Red Army mauls the WAllied forces, at some hefty losses to themselves, but stall out around the Rhine. Bloody stalemate ensued for 1-2 years until Soviet economic problems, overwhelming American build-up, and a whole bunch of nukes grinds them down and they start to lose ground at an increasing rate.
The fact that in this scenario it's the Soviets attacking first handily solves the the potentially war-losing political challenges WAllies would normally face if they were the ones attacking first.
This war will nonetheless still take years, see millions of Americans die, and leave Central Europe a devastated basket case.