I already like this story immensely, but it's even better on the second reading. I normally don't read much Snarry because there's often an unbalanced power dynamic between the characters, especially with Snape being so much older than Harry. This solved that problem for me beautifully, and I love that it happened without actually de-aging Snape, and also without actual time travel. (Maybe it's more dimensional travel?) Your Harry is so believable -- he's not an idiot, but he makes these very plausible mistakes (like the way he lies to Severus about who he is.) It's your Snape, though, that really captivates me. I love that he is very much the wary and damaged teenager we see in the Pensieve memories, but he's not yet embittered and certainly not unsalvageable. I also love how the story starts with Harry haunted by dreams of Snape's death and wishing he could have done something, and then fulfilling that wish through putting dream!Severus on a different path.
I also love the way you wrote the other characters, especially Ron. He's not usually my favorite character but I really enjoyed him here, especially his interactions with Harry.
Even though I'm out of the habit of reading Snarry these days, I'm so glad I read this one. It reminds me very much of the 'old school' Snarry fics I enjoyed when I first got into fandom. The whole story is well realized and beautifully done!