How sweet! This is very well written, and what a carefully crafted surprise.
I love how Snape takes Harry in - having already misled Poppy and Minerva. And all three of them kindly take care of him, their battered but beloved snarky Slytherin. It's a kind of H/C, which I utterly love.
This was pure angsty brilliance, when Harry almost flung out the door in anger, but Snape begs him to stay: As he looked into Snape's openly anguished face, he suddenly remembered a vision of a much younger Snape standing outside the Gryffindor common room, begging a red-haired girl to forgive him. "I'm sorry," Snape said again. Harry's ire melted away. "Okay," Harry said. "Okay.
Oh, oh! Poor Snape. He's been cut off by an unforgiving Gryffindor before - Harry's mother. The sainted Lily. It always bothered me that her friendship was so conditional, and that she'd abandon Severus even when he apologized. How very, very nice that Harry is more loving and forgiving! (At least in this fic; in canon, not so much.) This was an inspired piece of writing! This reference to others long gone, the tie-in to all that history of Snape's, and Harry's, and now shared between them, adds another wonderful level to the fic.