Hmm, I think I fit the bill 4, and just wanted to say it is a delight to read a post, however long, by someone who really has done some introspection and has so much of their shit together as you do.
To the issue at hand;
1) She has cheated on you. Trust issues are bound to happen.
2) The vast majority of cheaters are not bad, rotten people. Most are just really confused, as it appears your gf is.
3) What's the deal with this friend? Does she want an on-going relationship with them and is just really scared to ask for one, because that would considerably alter the dynamics of your relationship?
Oh, and hello, too!
Me: bi female in my twenties