I think at this point the US is probably doomed, unless it can halt the invasion in Canada. It can't use nukes in the amercias without compelte abandoning thier allies and reason, so mexico and canda have to be secured through force. If the US gets invaded, I think its the end for them. If it gets nuked, same thing. But I would advise against the russians doing that if they want any postwar relations with them.
Indeed. The US can mobilize their entire reserve force, however looking at Belgium during WW2 it didn't go as well. The Belgians mobilized all their reserves in a total defense, actually gaining the same numbers as the Germans, however a bunch of untrained and ill equipped reserves and civillians stand little chance against a well oiled war machine.
The US can of course nuke their allies or even their own cities, but as you said then they have lost. They might halt or even stop the Soviet offensive that way, but at what cost? Perhaps even falling to communism is a better prize to pay than nuking your own allies and cities. The Soviets also need to tread carefully. It is not as easy to control USA as lets say Poland. I guess it depend on what their aims are. A swift victory at all cost? Revenge for the US nuclear bombings? Or, as you said, wanting to have good or cordial postwar relations?