In the book, Maggie is actually Cody's grandmother. Also, the book has elements of reincarnation, Christian mysticism, Egyptian mythology, a real cosmic battle between good (as represented by Cody, Maggie, et al) and evil (Erik and Nick). The move is pablum, thinned down version of the book, which is infinitely better.
Everything should be done in moderation - even moderation.

Oh I thought she was saying Ma'am. I have a friend that calls her grandmother that. I thought it was a sign of respect.

I always wondered that too. I don't know if it means aunt in another language or something, but I thought that maybe because her aunt is her mother-figure but not quite her mother, so instead of 'mum' Cody calls her 'mim'?