What I really liked about this is how it shows how altering something in our past, especially one we're unhappy with, doesn't necessarily mean what we end up with will be a better one. Snape sorting into Gryffindor certainly didn't turn our, and having a relationsip, albeit an uneasy one, with the married Potter's turned out to not fix anything either.
The one where Snape tried to 'fix' his teacher/student relationship with Harry turned out to be the worst one of all, and I think it was this one that was the critical one, so far as Snape realizing that having this item or that fixed wasn't the crucial thing. He was set up by all of these, though, to realize that was his choice that was such a powerful factor, so far as his own destiny.
The last 'gift' was the one the reader was waiting for, of course. Waiting for Snape 'to get a clue', his willingness to 'play along' and see how the scenario would turn out, his gradual fascination with the Pensieve--he was already changed by those earlier visions, and one could sense that he actually realized this was his last chance.
I like that the story really ends with a possibility, but one that it's fairly obvious will happily come to past. Good ol' meddling Albus!
You did a wonderful job with this, picking just the right scenes, the 'if onlys' that seemed to have maximum effect, the very ones Snape must've brooded over all of his life. His and Harry's 'scene' was precious and made me sigh with relief; at that point, realizing this was still an 'if only', I felt fairly sure that both of them would end up in the real world with a happy outcome.
Seeking Knowledge was a difficult prompt, but I think you nailed it. Because Snape truly was trying to understand his choices and what could've been the diverging points; if Snape were anything, he was an inquisitive person who needed to understand why his life had turned out the way it had and what could he've done differently.
I really loved it, J. I know it stretched you as a writer and gave you fits at time, but you really pulled off a beautiful and satisfying fic!