I really like how you used the premise of memory loss to explore these characters, and character in general -- how our experiences change us, but there's also a deeper core of who we truly are. Even without specific knowledge of her life and choices, Minerva is absolutely recognizable as herself, and you can easily see the different paths she could have taken in life while still being her. I also think you're spot on with Neville, how he's matured so much but still feels dragged down by other people's knowledge of who he used to be. Though he doesn't lose those memories, being with someone who knows nothing of them frees him to be more himself than he's ever been, and it neatly fills in the gap of time in DH when we know he must have been having these life-changing experiences but we don't see them.
The other characters are also very well drawn, especially Snape. I think you captured his predicament perfectly -- the tightrope he has to walk, pretending his motives are other than what they are and not raising the DEs' suspicions.
At the end I did wonder if Minerva might have remembered more than she let on, like Odogoddess suggested. Or maybe I just wanted her to remember, since I do think that although Neville's actions were understandable and in character, it's a gray area ethically. I'd like to think there'd come a point where if she didn't remember, he'd at least tell her; I think she deserves to know, and I'd want to if it were me. But I guess there's room to imagine what might happen in the future. :)