Having a little chuckle here.. That Red Gambit series has a take on every question you asked.
I suppose my point was.. getting to Russia to drop the nukes would not be the task it was against Japan with a more hostile airspace. Plus there is the range factor. Pretty sure a German Ally would not take too kindly to having their country irradiated.. Germany would be the front line within hours of hostilities, so assuming the nukes were still in the USA.. getting to the theater would take some amount of time. Assuming that Persia or any Balkan country is "friendly" would also be a big gamble. You could be assured that the NKVD/GBH would have agents actively being subversive in most border country governments
War weariness would also be issue, as would re-mobilising demobbed forces and gear
OK Red Gambit Spoilers.
In the books hints and intelligence grabs of a build up lead to the Allies arranging and attending German led symposiums by proven German battle commanders to pass on their experience, tactics and give appraisals to the allied commanders about fighting Soviets.
As far as German forces, there are political forces in play. Ex Wehrmacht were quickly approved to be used as troops, as they are fighting on their home turf, so would resist anyway. But not the SS. Funnily enoug, when it got bad enough after Soviet breakthroughs, the SS forces became members of the French Foreign Legion which were seen to be conveniently "outside" of the Allied Political structure
