I assume you included America in with the Allies Gawain, but unless Russia did something to provoke America, im not certain that would happen.
Well for one thing Russia invaded Poland as well. They also invaded as many if not more countries than Germany did at the start of the war. Also didnt we fight the commies in the first world war? I dont think our government in reality was ever a great fan of communism or Russia under their rule. It was a matter of picking the lesser of two evils(on this I think they picked the greater of two evils to ally themselves with) but the one they saw as an immediate threat. Germany and France still had too much bad blood between them not to fight eachother it seems.

So either way - with or without the US - russia could have been defeated on her own.
Yes and this is what makes me laugh when people claim Russia could have beaten Germany alone. With the help of all the allies it still took them 6 years to win. The wonder to me is how did Germany last as long as it did with most of the Industrialzed world against it. One on one Mano on Mano its no contest.