Ooooh, now this is interesting and different and fantastic! Filch's voice feels so authentic, and you've turned him into such a three-dimensional character here. I've always wondered about Filch in canon -- what his family and upbringing were like, how he came to be caretaker at Hogwarts, why he chose to live around wizards rather than Muggles, what he does on a regular basis -- and you've done a brilliant job of tackling some of those questions. The details of his life and imaginative and entirely plausible, and do a great deal to explain his character.
I love that Filch is so perceptive here. He has such a vivid, different way of seeing and discerning things. His insights into Snape and later, the Snape/Minerva relationship, are fantastic. Through him, you've portrayed a very believable Snape and Minerva, and conveyed the nature of their relationship perfectly. Yay for Filch understanding so well.
I loved the idea that even a Squib might have some low level of magic within them that manifests unpredictably in certain ways. Filch's ability to know things about people is fascinating. (Although, one is left wondering whether it's really magic, or just great intuition and instincts that he wishes were magic!)