I adore this story, Mystery Author. Your Filch is wonderful -- perfectly in character, but very human and even likeable. His backstory is great and very believable, and I love how he isn't all over Umbridge just because she's (seemingly) reaching out to him. What we see of the Snape/McG relationship is lovely, too.
Thank you for this wonderful read!
(I'd still like to see a sequel where Umbridge tries to seduce Snape, though. *g*)