Just to clarify matters a bit, I do not (yet) have the revised version so I can only comment on the orignal A & A.
However, in the original A&A, nowhere in the rules do I find anything that states US or UK troops cannot occupy the same territories as Russian troops. Is this a house rule that you use? If this is the case then the Axis should win nearly every game as I cannot see any way that Russia would not eventually be overwhelmed. Also, I do believe that it is allowed to occupy neutral countries as long as you are willing to pay 3 ipcs.
A direct invasion of Germany is rare, but it can happen with the Uk/US double team and a bit of good luck with the dice as long as they are willing to sacrifice planes. The UK and US can fly all its fighters from Karelia (or Finland or even from carriers) to participate in this attack. In the mid to late game and without the African ipcs, Germany begins to get really stretched in trying to defend against a massive buildup against E. Europe, where most of the forces normally go such that it becomes really tempting to leave Germany and perhaps even W. Europe thinly defended.