I don't believe the Soviets could have struck back. the Germans were just too strong. They swept straight into the European half of Russia with next to no issue. Only American arms and support kept Russia propped up.
If that had not been there and if (I know, many "ifs") the Japanese had hit Russia in the back in Syberi, the Nazis would have ridden rough-shod straight into Central Asia.
What would have happened then is the same with any over-stretched Empire, especially one that over-stretches so quickly -- it would have broken.
Possible. However, the Japs tried it on with the Red Army in 1939 and got absolutely smashed.
The Japs were set up to fight the obsolete Chinese forces and were in no shape to deal with mechanized land warfare in open country. Banzai charges didn't work any better against Soviet tanks than against American marines.