You asked to hear from poly/mono partners who ended up getting a divorce. I am the girlfriend of a man who got a divorce. We were in a m-f-f tryad with me, his wife, and him for a while, until it fell apart. Well, hindsight is 20/20 so here is my two cents (keeping in mind 2 cents doesn't buy you much nowadays...
He felt he was always poly, has wanted it since he was 15. She had been in somethign poly before but then denied that aspect of her past and became very strongly Christian, as she had experienced much pain and heart break in her former life.
They had plenty of deep conversations about it. Tip: this was a good thing. They had managed to go beyond the obvious emotions of betrayal, surprise, insecurity.
What they didn't do was go to therapy. Tip: Try therapy with someone else who your wife actually feels comfortable with. Tip 2: Sometimes getting beyond feelings of insecurity and JUST CONVERSATION is what is most important. I so agree that it's NOT about sayhing "this is what we're doing right now"... it's about being able to support, understand, and HEAR each other's deepest emotions and beliefs. (more on that later...)
Their marriage was on the rocks super bad when they met me, for reasons that had nothign to do with polyamory and everything to do with extensive personal problems between the two of them. Tip: Yeah... if there are other things going on in your marriage beyond poly that make you question your marriage, poly will only make things crazier. (this one is so obvious in restrospect, it's painful.)
When my boyfriend met me, he felt like he couldn't hold it in any longer. He went back to his wife and the two of them talked again. By then, he was desperate for changing the situation of his marriage. Things got rushed.
Tip: Yeah, that's just a bad idea. Introducing the idea of poly is something that takes small baby steps, gentle ones.
Well, it should be obvious by now what our mistakes were and why our attempt at poly failed. We went way too fast, and their marriage was already on the brink of collapse. They had extraordinarily serious issues from the beginning of their marriage that had nothing to do with me, and that I could do nothing to help, but only emphasized to both of them that they weren't happy.
How have things gone since their separation?
He is in the middle of arranging 50/50 custody on paper; in practice, he already has 50% care for his child.
My family is NOT impressed with me, and doesn't support the relationship
His family has accepted his decisions, because his relationship with his wife was so rocky for so many years, every one understands
I love him dearly and he loves me
We are in much more careful conversations and practice of polyamory now having gained some wisdom and experience the hard way. We are much more careful, slow, and discuss EVERYTHING with each other before jumping in. It helps!
So... those are my reflections. I hope they help!!!