4. You can continue in all areas even after the MQ once dlc is installed ( I don't see this happening at all ).
The dev's prior to NV's launch made it clear that too many issues ( whether gamplay bugs or story based ) would pop up afterwards.
yes, they would need to cover all possible endings if they removed the game ending. It would require to much resources and of course it could be bugged and so on.
But I like your first idea. I have seen it happening in Dragon Age Origins. You can load the save after the final battle and you appear in your camp. When you open the map, you can't travel anywhere but DLC location(s). I'd love it to be implemented here as well