found Harry by far the least sympathetic That's as it should be, I think - the person who's actually doing the cheating is the one that's most in the wrong - but I wrote it from Harry's perspective because I wanted him to be at least somewhat sympathetic; anyone else's perspective would have made him come off even worse.
I'm very happy you liked the portrayals of Snape and Ginny. Because of the Harry perspective, I was afraid they wouldn't come off as 3-dimensional -- Harry, in canon, isn't the most reliable narrator. Snape was easier because I actually see him in 3-D. Ginny, OTOH, is pretty 2-dimensional in canon, but for this fic it seemed really important to me that she not be portrayed as a harridan or empty; she needed to be as 3-dimensional as Snape and her relationship with Harry needed (for me, anyway) to be a good one, a loving one, because how else to explain 20 years of marriage, and I was going for as canon-compliant as possible.
Thanks so much for the meaty comment and, of course, for taking the time to read the fic.