The Russians seized the initiative that first winter with a combination of the influx of lend lease aid and very capable and cold weather equipped Siberian troops. Those troops were freed up from duty in the east because Russia had just signed a secret non-aggression pact. I think it is very arguable that Germany would have had a chance on the eastern front if those troops had not arrived. There is also the argument that if Britain had not fought and slowed Italy in Greece then Germany would have launched their invasion of Russia earlier and perhaps taken out Moscow before winter set in.
I am not so sure that the Allies aid was all that big a factor in late 1941. The supply systems were just getting up to speed by that time and a fair amount of what did arrive was not suited for use in that theater.
As an aside the three best things that the Allies supplied the Russians in WW-II were 1} Studebaker Trucks 2} Waterproof phone wire and 3} 100 octane av-gas.
In late 1941 General Winter was the Russians biggest ally in the “Great Patriotic War”.
And you are correct that the secret treaty had freed up the Siberian troops for use in the West. But treaties are made to be broken and even with the troops being sent to the West I doubt that the Japanese would have had much success in the long term.
Also the Japanese had tried pushing the Russians around in 1940 and almost had their heads handed to them so I doubt the Japanese were in a hurry to try their luck again.
Regards
alfa6 ;>}