This is completely wonderful. So many great things about it I can't even touch on them all, but you definitely had me at Today, he was walking right into the arms of the enemy.
I loved your descriptions of Snape throughout, especially in their first lesson -- all oily blacks and ghostly whites, a hunched caricature drawn with a broad brush. Perfect.
This is definitely going on my recs list, and I'm sure I'll be re-reading it many times. Thank you for a great read!