I like the use of Aberforth here; he mostly seems to be either ignored or used as a vehicle for cheap goat jokes, so it's nice to see him actually developed as a character, with his own personality and agenda. And I can believe him and Snape being friends, irascible semi-outcasts as they both were.
I have a real soft spot for 'Snape gets a second chance at life' stories, so I was already primed to like this one, and I enjoyed the way you developed the theme. I also liked the relationship between Kieran and Harry, and especially the fact that while there was tension and misunderstanding, there wasn't a lot of heavy angst.