If Hitler had left it to hid generals Germany absolutely would have destroyed Russia.
I don't think so.
The Soviets had the USA as an unlimited source of raw materials, petroleum, ammunition, tanks, aircraft, trucks, foodstuffs, etc.
It would be impossible to defeat the USSR.
Don't think so. The German Army was at the end of it's rope by the Winter of 1941. All momentum had been lost in front of Leningrad & Moscow. The units were operating deep inside Russia with few improved roads & rail connections with which to move supplies.
Hitler's scheme to sieze the oil fields of the Caucuses when offensive fighting resumed in 1942 showed that there were too many objectives and insufficient resources. In addition the Lend-Lease supplies were arriving in Soviet ports in vast quantities.
Lastly, Stalin knew by then that the danger posed by Japan in the Far East had passed when Japan "Struck South" after Pearl Harbor. He was now free to shift his Asian troops to the West.
The Russians had even planned for the loss of Moscow, so even had the OKW persuaded Hitler to go for that strategic prize instead of Stalingrad, it wouldn't have made much difference.