This is the main question: was the Lend & Lease policy the main factor for the Soviet Victory in the eastern Front during world war 2? I’ve read and heard here and there that the US Lend & Lease policy greatly helped the USSR in its fight against the Wehrmacht. But when did it started? After the Germans were stopped at Moscow and Stalingrad during 1941 - 42? Or when the counterattacks had already started in 1943? Or later? Or earlier? How much did it help? If it started before the battles of Stalingrad and Moscow can I assume this is the main factor for the Soviet victory? Etc, you get the point. Thank you in advance.
While it doesn't address all of your points, I have written extensively about Lend-Lease and comparison of US-Soviet produtcion before, which you can find here, and some of the follow-ups do get into the other parts you asked.