Wonderful story. Mostly I appreciated the way you told it -- layering in tidbits of narrative until it all wove together into one string of engaging storytelling. I love when stories build like that. Awesome craft.
Now to the story itself. I loved all the characters and their various roles, the openness and authenticity with which they communicated with each other, the way this snuck right into canon seamlessly (we are happy to be none the wiser!), de-aged Snape (one of my favorite things), and, of course, the sexy bits. Even Dumbledore, who I think is a difficult character to write, you managed to nail with his almost-but-not-quite-a-riddle advice. Bravo!