- I think it's referenced (as well as being stated in the book in which she was introduced) that Millie has to/will have to periodically return to her own world in disguise. I don't remember about Mordecai, though.
- Given that Mordercai was pretty much bought by Christopher, it's possible that he is a special case?
- Wasn't Millie's crossing paid for with one of Christopher's lives? That might well give her a pass.
- She used one of Christopher's lives to cross from one world to another, and later on Gabriel gave one of his lives to Asheth in her place. Presumably one way or another her connection to her original world is broken.
Headscratchers / Chrestomanci
Moved from main page: So, Conrad, you say no person can live long-term in a world that's not their own? How strange that no one remarks on anything bad happening to Millie or Mordecai, then.