I'm coming at this from the POV of the reluctant mono. Wife surprised me with non-monogamy and, like your partner's GF, I felt like I was trapped between a partial relationship or Zero relationship. I chose partial and had to struggle with a lot of resentment. Some of which I'm still dealing with.
There's a reason so many "Beginning Poly" resources strongly advocate that you should only accept a polyamorous relationship if YOU really want to. Not to try to hold on to (part of) an existing relationship. GF has made that mistake and it's likely not going to work out for her. So in my experience, just waiting for her to "pull through" and be okay with polyamory is unrealistic.
Also, I'd second sdguitarguy's concern about it taking a few months before you were aware of the situation. Bit of a red flag, that. I'm wondering if you've had the opportunity to communicate with GF directly? Maybe she'd be less prone to vilifying you if she were able to see you as more than a "home wrecker".
She may not be open to that but it's something to try. As things are going now, this is going to be a messy situation. Esp if marriage gets involved while she's reluctant.