Awesome theory. I happen to be listening to Cold Days and James Masters delivers Lacuna's line about candy on Halloween with disgust, followed shortly by her flinging Toot-Toot's gift of candy into the garbage disposal. Also, the hooks on her armor could be the heads of the dentist scraper's designed for removing plaque.
I'm pretty certain there is some text with Harry narrating about the Sidhe and explaining that the tooth fairy is real, he knows her and she is seriously scary. Harry has also narrated about Santa in a similar fashion, and we've now met him, so why not the tooth fairy, too?