Oh. My. God. I'm just in awe of this story. Whoever you are, mystery Santa, you must know me and my tastes very, very well, because this is everything I could have wished for in a fic. Everything. Excellent plot, angsty twists and turns, a hopeful ending that leaves the bulk of its promise to the heart of the reader and characters with complicated motives.
Things that made me fall in love with this fic:
Severus' room. I love that he had to be in the right frame of mind to access it, and that he was never sure, day to day, if he would see it again the next time he tried to access it. I love the symbolism of it, too, which mirrors his character so well: the room, locked away with uncertain access, where he keeps his most precious memories, is just like the very private man we know from canon, who keeps his memories locked away safe until the very end.
Which brings me to another really amazing thing about this story -- its seamless joining with canon. These could be lost scenes, they're so tightly woven together with the canon story. In canon, Voldemort never recognized the love Snape still held for Lily, and that made him blind to Severus' disloyalty. It's these kinds of little puzzle pieces you've made that fit so well with canon, which make your story so rich and believable, and so very, very enjoyable. I couldn't take my eye off the page, except for one point, when I got out OotP so I could follow along, and fit the scenes in (yes, I was that excited about the story!).
I was also very impressed with your delineation of Snape and Lily's relationship. I especially liked the vignette where Lily is asking Snape again and again to talk about the Dementors. I think that really rings true; she feels something in her, the magic, maybe, that drives her to know more about her world that she doesn't know about, and I can see her being that headstrong, pestering Severus until he tells her what she wants to know. These scenes of his youth with Lily provided the perfect counterpart to his anguish at her loss, to his obsessive and unhealthy behaviors with his memories of her. They make us feel his losses even as he feels them. This was very well done.
Snape... wow. You've shown in painful detail what a horrible position Dumbledore's put him in. He's ripped in two, maybe ripped in four, even, and your storytelling ability has us bleeding and trying right along with him. *swoons*. To ride for so long that thin line between seeming to serve Voldemort and doing the right thing by Harry, and then the way you show him trying to cope, like the fracturing man he is... wow. I bled with him. And in the end, being forced to do what he would have liked to have done in different circumstances, to have to live with that lie and still he did it, just rings so true with canon Snape. *melts*
And Harry, oh God, poor soul, he leans in and takes what he wants, he finally gets Snape, and then -- after OMGHOTSEX -- Snape comes back to his rooms to find Harry amidst all the memories of Snape taking him, raping him... when Harry says "Take them...all of them. Last night..." Potter paused, his voice breaking. "I want to forget." And Snape, after giving him everything, to the cost of his safety, his position as a spy, his morality -- he gives Harry this, too. I was just devastated. I could feel Harry's hurt and I could feel Severus' heart tumble down, down, and I felt like I could cry at the tragedy of it all.
But then, the Epilogue (and can I just say I like that it was 20 years later?) :-) No lengthy explanations, just "anti-venon", and really, that's all it takes to explain it, doesn't it, and they have a chance to start afresh, with everything back safe in their minds. Just that chance for the tragedy to be repaired, was the perfect ending. Just perfect.
Mystery author, I can't wait to find out who you are so I can go read everything else you've written. What an amazing, amazing gift.