I think one shouldn't look at NTR stories from perspective of best course of action/ what should he/she do or what is the most probable thing person in such situation would do . It's best to look at things from a "is it possible" point of view, are there behavioral reasons for someone to act like this. ( Not to mention strong emotions do make people act irrationally.)
So yea it's easy to say that "Harry should just move on" , we know that ain't so easy to do for a heartbroken person. Also by that logic Laura could have decided that Harry ain't worth the hassle and he should answer for his own fails. But then there wouldn't be a story there right?
For good ending - obvious good ending would for the whole thing to not happen - either Harry not trusting Luca or Laura preparing evidence for court. But such ending would be lame and not much better then not reading the story at all.
Good ending will be when Laura will chose Harry over sex basically. One course would be that Harry basically never learns about the whole thing and Laura deals with Luca herself. Or Harry do learns about it and understands that what she did was for him.
Wicked good ending would be where Laura keeps fucking others behind Harry's back and Harry just never learns about it. - technically both would be happy
I'm most curious about Harry and Luca's past and does Luca's mom play role in it. We witnessed twice Luca having strong defensive reaction when his mother was mentioned. I don't think HC put it there for no reason...