[Dac X Lee]

All interesting questions.
The "did you ever forgive her", to me always felt like Nicky simply referred to Richey in feminine pronouns because, well, for obvious reasons. And it wouldn't be the only time he did that either, wasn't Tsunami about Nicky and Richey by making the parallel with the twin sisters as the motif of the song?

Hooooowever....
Now that we have that posting about that infamous book about Richey, which mentions that mysterious Vivian, and, as it was initially presumed Jo was this "girlfriend he broke up with".... Hmmm.... Good conspiracy material.

Edit (text added)
Or maybe the guilt is what Nicky feels in regards to his friendship with Richey. Maybe he feels like he didn't do enough to help Richey overcome his health problems, maybe he feels guilty for not being able to catch him at the moment of that unfortunate day. So maybe the feminine is Nicky addressing himself and asking himself "Will I ever be able to forgive myself?"