Ah, now this is the Beholder I love: dense, meaty stories that take time to build world and character, all the while being fully canon-compliant. You interweave both pairings excellently, with believable parallels and such strong characterizations of all four people (and of Albus, too). You've rendered the main characters so distinctly -- Argus really comes into his own, and the Snape/McGonagall relationship is just the way I like it: snarky, dry, understated, yet tender. And the way you play with mother/son issues (on several levels) is interestingly complex and revealing.
I also love the use you make of the Irma Pince/"I'm a Prince" theory. I first read that idea not long after HBP came out, and I thought it made wonderful sense. Would that canon had dealt with it as well as you have.
So many fun lines, but I particularly enjoyed this one:
There were, she realized, far worse things for a lady of a certain age to do with her summer evenings than go out for a very nice dinner (with an equally nice bottle of wine) on the continent with a junior colleague, one possessed of a certain disagreeable charm, striking intelligence, a sharply-tailored set of dress robes, and a very shaggable arse. Haha! I do adore a wonderfully pragmatic Minerva.