What an absolutely beautiful story. The plot works wonderfully well - and that's no mean feat when you've messed about with time. Thank heavens you're more skilled at it than Severus.
Your Filch is a delight. Perfectly IC, but with so many more dimensions. The practical way he solves the first set of problems with the boy. And every other problem after. No wonder the school nearly fell apart with him gone. And the rough-and-ready way in which he teaches the boy his manners. And Snape, too. The apologies-scene is terrific.
And Snape's painful honesty (no-one would miss him much) and slow acceptance of the annoyance are just as perfectly depicted.
But Little Severus steals the show. It's so difficult to get a child's voice right, and you got him to perfection.
A wonderful, beautifully-told, briliantly-characterized dream of a story. (Occasionally one is allowed to go over the top with adverbs. I won't make a habit of it.)