Edwin going along with the Rosalind threesome on the Hana GF path is a matter of two things:
1) How he perceives it to not be a violation of his promise to Hana.
2) Me not wanting to write a messy exit path that will complicate the boys' relationship for the small cross section of players that did the threesome path and then then GF'd up Hana. It's a hackish reasion, but I can't say that didn't factor into the choice of keeping things linear there.
Realistically, Hana would / will take umbrage about the situation; if not on a fidelity basis, than just how messed up Ian taking advantage of Rosalind's debt to have some "bro time" is and Edwin's lack of compunction going along with it. The thing is, that is an extension of Hana's problems with the club in a general, and a reflection of her own disappointment in herself for abiding it. Her getting mad at Edwin would be a form of self-loathing, even if she's not aware of it.
Food for thought: the whole "monogamy" thing is a tenuous stipulation to begin with, and even if Edwin keeps to it, both acknowledged that it still may not work out. Saying you're okay with your boyfriend fucking whores for work may sound good in the moment, sound like a logical concession given their reality, but when you witness it, or feelings develop deeper... who knows. Feelings are messy and relationships aren't build on logic. On the flip side, it could work out; they have a rather unique connection, in the midst of volatile situation, and player choice will factor into how their relationship progresses.
Also, some hack is controlling their lives like a puppet