I always forget the mirror dimension is a thing of its own. I guess I just think it makes more sense for it to be (connected to) Dixing--maybe when you go out the door in the mirror apartment you're in Dixing, and the apartment is just its own weird zone?
(And I do kind of think the magpie wish is just Changcheng being Changcheng, but then that's something I have a lot of time for ;) . omigod, imagine accidentally wishing on a Crow Yashou, that would not end happily.)