Such a fine story and such revealing artwork. I'm really taken with this entry; it's one of my favorites in the fest so far.
When it comes to Argus, I'm like Albus here -- intrigued and fascinated by the mystery of him. I love the way you've brought Filch alive as a complex, real person and not the way he's presented in canon, as mostly squicky comic relief.
WWI history is another kink of mine, and I'm impressed by the context and backstory you create for Filch. Like so many soldiers in war, he's simultaneously victim and perpetrator, and his background gives such depth to his character. As preverse_idyll says above, this bond of . . .well, I suppose it can be called "evil" between Dumbledore and Filch works very well, both as an explanation for their connection and as a means of providing equality to their relationship.
Albus is beautifully-drawn, too, both in the art and in the fic. You capture that effective canon sense of him as both well-meaning and yet often blundering. And the sex is so compelling because not only is it erotic, but it works so well to continue the characterizations of both men.
I also love stories that provide satisfying explanations for canon events -- such as why an elderly Squib is the caretaker of a magical school.
Among the many great lines: Magic literally saturates his being. He can tickle it with his fingertips and taste it in the air when he closes his eyes