I'm really pleased you liked this story (I haven't written Snape/Regulus since about a week after Regulus was first introduced in OotP, so it was fun to be able to revisit the pairing), and particularly pleased that the youth of the two characters seemed to work. There's a tendency to write characters who are smart (like...Severus, for example) as older than they are, which yes...in some ways they are, but smart or not, these are still teenage boys.
"I like to imagine that part of the reason Regulus started to doubt Voldemort (besides what happened to Kreacher) was that he had second thoughts about what he did to Severus..."