Oh, there are so many things I loved about this story. The idea that Harry's flipping in Dumbledore's office mirrored Snape's, when he was 15, was beautiful. It's a reasonable explanation how Dumbledore managed to get Snape to keep quiet. I loved how Dumbledore was already worried about Sirius's loyalties. Well, of course he must have been, but it was really nice to read about it.
Plus you mentioned Regulus as a friend of Severus and his protective nature, and the conflict between his loyalty to Lucius and Dumbledore, which I expect to be addressed, somehow, in book 7. And I think you captured Dumbledore to a T.
Have you pimped this in a community?