And Hitler's pride. Up until then the Germans had (correctly) bypassed cities of resistance (e.g., Kiev, Leningrad) and instead isolated them and laid siege to them. Both Hitler and Stalin turned Stalingrad into a political strategic point of victory.Stalingrad was not a strategic point, just about Stalin's pride. The Caucus oil might have been significant but that still would have been long and vulnerable supply lines. The Russians had enough numbers to continue fighting and reestablish production facilities further east. Without a significant increase in manpower and resources, Russia would be unlikely to be conquered. Even a Japanese invasion into the east would have suffered so much from the weather and distance to be ineffective (as they were a few years earlier).
Whether the USSR could have been defeated if the Germans had done things differently is total speculation as we'll never know what other events would have transpired and played out. What is clear is that the German planners dramatically underestimated the reserves that the USSR could call upon and also the average Russian's willingness to fight (and die) for Mother Russia. Combined, these two estimations had fatal consequences for the Germans. Was the USSR unbeatable? I don't think so, but I would say the odds did not favour the Germans. Had they taken Moscow in 1941 things could have turned out very differently (as it was the central artery for transportation in western Russia, not to mention the political heart of the USSR). That wouldn't have ended the war but it would have made it much more difficult for the USSR to conduct.
I agree about Japan. If Germany was ill-equipped to take on the vastness of the Soviet empire, this is doubly true for the Japanese.