Wow. Not too coherent right now, but I had to tell you that is an amazing feat of characterization, and I am just bowled over by it. This is probably the most authentic Filch voice I've ever heard - not just the dialect, the the thoughts 'spoken' in it. They conjured the Hogwarts caretaker up completely for me, as real as could be.
The page I'm commenting on doesn't have your text in front of me, but there are many passages which struck me, and which I can remember without having to look back (but I'm not guarantee-ing that I'll quote them perfectly ;-) ): --The bit about Argus's Uncle Stan not 'holding with implements' but that wizards were harder and meaner; --Severus's voice being ultra smooth when talking with Umbridge, and Argus knowing that is a danger sign; -- Argus being a wee bit freaked out by the cutesy kitten plates; -- that Fudge treated him as if he were a servant who 'wore turbans and had to salaam to his 'betters' ', and that he thought it 'unseemly' for Umbridge to act the flirty girl around Fudge; -- that what magic Argus has shows up as natural Legilimency; -- Argus's discovery of why the kids call her 'The Toad' (omg, tongue-flicking, eeeew!); -- the manacles Pringle gave him, and equating them with magic and magic with power and control; -- that he understood Umbridge wanted Severus because 'Severus were a man who made people feel strong, dark and needy things' and that Umbridge wasn't 'the first person to want him. To want to understand him or save him or join in his righteous darkness. To want to share the power of him. To have him want them.' (*This*) -- your description of Filch's fantasy of Severus in his potions lab - eep! It wasn't just Filch who saw it before his eyes. (Oh, and Mr. Pumblechook is a *fabulous* name for his cat ::g::) -- the comparison of orgasm with being jinxed, and not 'holding with bodily fluids'; -- how he understands his voyeurism in terms of the way one of his aunt's cats behaved and that touches are 'like little fires' and 'too much'; -- the Castle making mini Rooms of Requirement for Filch when needed, whatever those needs might be; -- Filch going from hot to not while sharing the crushing boredom of marking papers (teach, do you? ::g::); -- Severus's distaste at giving in to his own needs, and Filch's excitement and desire to stay quiet so that he wouldn't miss any of the sounds Severus was making; -- how Argus finds doesn't mind finding Severus with someone, because, since it's not a man, he can remain emotionally removed, - then nearly being 'killed straight dead' at the sight of Minerva McGonagall in Severus's bed; -- His sentiments concerning McGonagall have such a ring of truth - this *is* how he would view her - and his reaction to Severus kissing her, watching his tenderness through Argus's eyes . . . I love how you expressed it as watching a mask peel from his face and being able to see the real Severus underneath for the first time; -- that incredibly lovely scene of the Minerva and Severus with The Daily Prophet!! Loved that whole thing, and the hot make-up sex scene, too; -- the poignancy of Argus's last look, with Severus's insecurity and paranoia on full display, and Minerva's (mostly) successful attempt at assuagement, and how Severus signals his acceptance of it by touch - tenderly removing the pins from her hair (and Argus recognizing that the pins are there solely for Severus's benefit); -- Argus calling Umbridge 'High Inquisitor', rather than 'Headmistress' - he's made up his mind where his loyalties lie.
Oh, holy cow - I think I've just about quoted your whole fic back at you! ::facepalm:: But really, this is such a tour de force of characterization that I couldn't resist detailing everything which makes it so special. I am floored (and more than a little envious) at your ability to leap inside this character's head, and make Argus Filch a human being whom we can understand, and even sympathize with - no mean feat, that. Brava!