first off, welcome to the forum. I apologize that I won't have time to respond fully at this time, but I'm sure someone else will jump in too.
There is the possibility that your gf will come around eventually. I'm personally a beleiver in people being able to change things, including their core beleifs.
It also might be that she can't, or won't, and you'll have some tricky decisions to make.
My personal thoughts on this is that poly shouldn't need to mean choosing one over another like the classic mono pattern of broken homes and high legal fees. It's more a matter of finding compatable partners...and no BS it can take some time and effort, and alot of patience to find such creatures.
By all means, try your best with the gf to see if there's a life that you can build together, and what that will look like, including the effects on the wife and families. But don't be too beholden to her for it. Especially in the throuhs of NRE, it's easy to make stupid decisions that you will regret later.
It may be that you won't be the person who can provide what Bree ultimately wants or needs...a stark realization, but a possibility you should reckognize if you really wantbto do right by her. And if that's the case the best thing you c,an do is let her go find her husband and 2.5 kids, and find yourself a partner that's more suited to your situation.
Well, that was more longwinded than I suspected.
(oh, and if it's any help, it sounds like you wife is a keeper...but just because she's willing to step down from primary, doesn't mean it should happen.)
Patience my friend...and good luck.