Although it’s a tough genie to keep in the bottle. Should they determine that DC folx have a 2A right, it will be hard to argue no one else does, and if they rule the DC folx lack any 2A right, it will be hard to argue that anyone else has one either. So I don’t think they can keep the scope of the decision as narrow as they might wish.
Not difficult at all. DC is under direct federal jurisdiction. They could restrict the federal government's power to regulate firearms without touching the state governments' power to do so.