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 ;>}
Right as things were most dire in 41 we some emergency shipment to them. Now those were not as BIG as later stuff. but we sent supplies and things when the were extremely critical. The first aid was actually from Brittan even before we joined the war. And we sent machinery and locomotives to help them move their industry to the Urals that winter. On paper those first few shipments were not huge, but the Russians stood on a knifes edge at that time. Even a few supplies, just in time, made a big difference. IMHO. Following this thread a couple years ago changed my opinion on that.