Oh, this is so good. Just delightful. I adore Argus's voice, and Madame Sally's, and your depiction of their old age, and Filch's solutions to Snape's temper, and. . .just all of it, really. Your writing is exceptional; so many fabulous lines. A very few of my favorites:
At 53 years of age, he was still in his sexual prime, which was more than most Muggle men could say for themselves. There were a few things, however, that put a slight damper on Argus's enjoyment of his sexual prime. -- I laughed aloud at this perfect use of repetition.
grumbling about one thing or another ("one thing" generally having the name of "Potter" and "another" mostly going by the name of "Black")