Oh, this is just a gem! So carefully and tightly crafted, and Irma is just a wonderful character (and so is Silvanus!). And their relationship is just adorable. You left me convinced that they were both lucky to have the other, which is a very nice place to end.
But yowza, what a clever plot. Of course Riddle's researches (and others') would lead him to try to cover his tracks, and naturally someone as formidable as Irma Pince would take exception to that.
The first thing that struck me was how you create a really compelling version of a magical professional; you translate the techniques and concerns and daily tasks of librarians into the magical world so carefully and completely---it was a delight to watch you work that through. And all the charm creation technique and theory was just marvelous.
And the end, ahhhh. Complicated, opaque, and clear enough what happened, on most levels (but open-ended enough to be make me think). Not hopeful, that---it can't be, given what we know is coming---although I'm very glad that they both survived the encounter with Riddle.
I wonder what would have happened if she'd confided in Albus?
Thank you for this, mystery author. I really enjoyed this.