You are right about this points, but this worked only so long, as germany needed the SU resources. You only have to read the infamous pamphlet of Mister H. to know it.
And in this game of alternative history are we only supposed to believe in military, technological and strategic differences from history or are political and ideological differences also possible? In a game where you play something of combined mind that controls military and political leaders then couldn't you have at least some influence on the political and ideological output of those leaders?
In the situation say where Germany is in a war of attrition with France and UK without making advances. Then
#1 Germany would be desperate for a secure border in the east and any help that it could get. At that point avoiding a war on two fronts would be essential.
#2 The Soviets would admittedly probably be fairly keen for that war to go on for a while weakening the Capitalist countries of western Europe. However, a plausible situation is that they agree to help Germany a little in exchange for getting pickings over British colonies / territories in Asia and/or parts of eastern / northern Europe.