The plot of Mirror Mirror sounds like a good story idea, but it also sounds like a great way to give Harry advance notice of information about his own world that he would otherwise not know. It gives him an edge in future books.
Example #1: What if Harry got involved (during mirror mirror) with the government agency focused on the supernatural. He would find out some of their leaders, some contact information, locations and maybe who their local "man" on the street is -- which by the way I think is Stallings...
Then in some future book, Harry can just call their chief of operations by name and number and demand assistance. He gets to freak them out (how did he know???), give off all sorts of "I am a wizard and I have mysterious sources of knowledge" vibes and get some great verbal moments. Something similar to when he messed with one of the einjaren at Marcone's place at the end of Skin Game -- using knowledge he gained as a ghost in Ghost Story.
Alternatively, Harry can find out some secret about his companions. Maybe one is sick or has an unusual relative.
Alternately, Harry can identify one of his companions as nemfected, with obvious implications.
Any other good opportunities for Harry to gain knowledge in Mirror Mirror in order to look Bad-Ass Wise in later books back in his own world?