MK and the Grand Floridian are connected by monorail so no need to move the car.
From Epcot to the Boardwalk is really easy if you go to the International Gateway, it's next to France, you can either walk through to the Boardwalk or catch a ride on the boats that go back and forth between Epcot and the Boardwalk hotels - not sure about Boma and AK but I'm sure someone else will be along soon[msnsmile2]