Yeah, some things why this should be absolutely impossible;
- Germany should not be able to even move past the UK if not conquered yet, hence, whole fleets with transports even supported by the entire KM should get into huge fights with both USA and UK, thing is, you don't have to DOW the USA until you are in place to land on the eastern seaboard, so US navy pretty much stays out of it, but RN should aggresively follow the KM and sink them, Germany never had a real chance at sea. Seeing an invasion fleet should have USA DOW Germany unless transports are pulled back, an event checking German transports closing in, something like that.
- Germany would never be able to create enough transports that can ship such amounts of troops over the Atlantic, I think that is still a problem in 2009, look at D-day, it's just too easy with those transports, it should be more tech related what you can transport and how much. Same for supplies reaching that far.
- On invasion, a huge but maybe less well equipped US army would be raised in weeks to stop the advance, this should also be event driven, at least militias and many many reserve units coming up.
- Supply ships would not survive the way back or over there, hence, a complete runout of ammunition and fuel.
- USSR would invade within weeks with full force
All these variables kept the USA save, and the UK for that matter, it just worked by chance and luck in Norway, I read that Hitler wanted to withdraw from Narvik after the UK landings, but it was Brauchitz that cooled the nerves. Look at what Germany lost in ships only doing that invasion.
Conclusion, RN and USN should probably be far more aggressive patrolling so nothing can slip through, historical belt around europe. That would hinder it.
I'm not too worried about it in game play now, as you have to take in mind the USSR...