This is masterful writing...the control of his voice, the internal consistency of his character, from first to last...It's just lovely.
You have filled in sooo many of the gaps in the story that JK Rowling has invited us to fill with our own imaginations, and have done so in a manner that does no damage to the characters involved. Your Lily is a girl worth dying for...she's creative and fun and smart, but a bit at a loss with Sev's brokenness, tho' she tries. Your Severus is quite simply one of the best takes on this character I've ever seen, unflinching when examining his scars, his essential darkness, but also showing the pride and courage that are, at base, him. And James, for example, is every bit the thoughtless boy who could grow into a decent man...so believeable. Many authors find that leap most difficult to make.
I think I've read a thousand fanfics in the past 5 years. This is easily in my top 10. I think I will bookmark it for those days I just want to sit and have a good cry. It is as cathartic as the best tragedies!!
You are truly a gifted writer, and I wish you much success with your original fic. all the best, editmom