I burned myself out on good!snape mentors Harry fics a while ago, but your first chapter caught my attention. I can see post-HBP Snape treating his younger pre-SS self as a dunderhead student who's about to pour the completely wrong ingredient into an already volatile brew.
While I can't see Sirius and Severus ever behaving cordially to each other if put in close quarters for an extended period of time, you at least made such a situation semi-plausible.
I caught your last update on ff.net. You're almost done!