OMG. This is absolutely one of the best stories I've ever read. If I sat here and quoted bits I'd go over comment limit in a heartbeat. So let me try to summarize.
There are so many awesome bits of phrasing and imagery in this, so many wonderful snippets of Severus's character and appearance and desirableness and Severusness, every so often something howlingly witty.
I love Lily's journey from hatred (which at the start had me spitting with defensiveness and wondering if it would ever dawn on her how much she sounded like Petunia condemning her "freaks") to, if not love, then at least understanding. The point where she starts referring to him as "Sev" again is certainly telling! You made Severus work hard in this story too, but somehow it's cleansing.
A couple of specific bits I'll indulge myself in:
Three pages of Hanon finger exercises, and the door bangs shut.
No wonder! God those things were bastardly. And not very much fun to listen to. Who can take minute after minute of constant arpeggiating?
"I'm afraid so," Albus says gravely. Tears sting Lily's eyes. "He has conducted himself with greater loyalty and courage than I'd any right to expect, and has paid the price. Dare I ask whether you've forgiven him enough to welcome him home?
Aaaaaagghh! you mean Albus orchestrated all this in giving her the "remote control" in the first place??? I don't know whether to want to punch him, or shriek with joy...
One thing. The Horcrux destroyed in the forest was the locket, wasn't it? You wrote it as the ring and it totally threw me.