A great story. I loved all the little hints of the differences in "this" Severus's world -- the Minister of Magic, Tom Riddle?? Sirius dead somehow, but Regulus alive, and Lupin still coming to teach in what would have been Harry's third year (but his sort-of-sister's first year). I'm so curious about other facets of the world as Severus knows it, but I like that we got tantalizing hints rather than out-and-out answers. Still, I would love to know about what Snape gave Regulus in exchange, and whether it might impact their reality unexpectedly as a result of Harry's visit/return.
One tiny thing -- you mention Lupin saying James had had a bad sleep/dream the night before, and then Severus reflects perhaps *Lily* can sense somehow her son is in this reality -- did you mean James in that following paragraph about sensing their son, or maybe instead Lily in the preceding one as having the dream?