As for the mirror reflection, the Phantom is known to be a good magician as well as musician, and the film makers actually show Raoul find the mirror slid open, and try to go down the same path Christine had taken, but he is stopped by Madame Giry. Though I will note the arms holding torches which were on the wall when Christine and the Phantom walked to his layer, were not there when Raoul walked in. This shows the mask switch could have been imagined, however the whole story is based on a book which is based on a real event. If the book is based on an actual maniac who lived in the Paris Opera house and killed people, I'd imagine the Phantom would be real in the movie and play as well. The book created a plot by adding romance, however I don't think Gaston Leroux wrote it as a dream, I think he wrote it as a fictional base on a real event.