This is very nicely done. I almost wish this wasn't part of a Snarry fest because I was able to deduce who "Lily" was from the beginning... but then I probably would never have read such an original and intriguing story!
I loved the way you took us slowly through Snape's brewing process. The Polyjuice Potion felt real to me for once, as if it's something I can grab a recipe for and attempt to reproduce. And Snape's relationship with "Lily" and Harry through different points of this process was also very interesting to read. He definitely accepts them both, and might even love Harry as much as he's fixated on Lily.