That just sucks. Sorry you are going through that.
Good relationships require trust in any configuration- I would say you she need to concentrate on re-establishing shattered trust first if your relationship is going to survive and be fulfilling.
From what you wrote, it sounds like there are much more important issues to deal with here before any discussion of polyamory is possible.
I hope you'll keep reading here on the forums and hopefully find some things you can relate to. Fulfilling love relationships involve trust, communication, and mutual consent if they are going to have a chance in hell, and it doesn't sound like these elements are present in your marriage.
"Rocks will open and make a way for the lover."
~Hazrat Inayat Khan
I love Catfish and Charlie.