I admit I feel bad about how things turned out for Si, but she did know things were on shaky ground. From Matt's writings, I could tell it just wasn't going to work out to have Si in your life. Plus her recent outburst and purposely pushing Matt's buttons doesn't help her cause.
I think it is counterproductive to try to figure out whose fault this was. No one was at fault or everyone played their own role in the situation. Mostly, I just think it was an unfortunate situation for all concerned.
Si is her own person, and can make her own decisions about how to live her own life from here on forward. Perhaps she will find it best to move back to the London area, if she has a support system of family and friends there.
You found that you had made an error in carrying the poly thing on for too long and too far. But it was not an error made maliciously; you did not mean to hurt Matt; you were just caught up in something and not realizing the extent of damage it was doing. As you came to realize it, you withdrew from the poly situation. We can't go back and fix the past, we have to focus on the present and do our best there. No doubt we will always have errors we become aware of as time goes on. Hopefully we correct the errors when we become aware of them.
Things did not work out perfectly but they seldom or never do in life. I have no ill will toward Si or Matt or you or your daughter or anyone else in this story. I just hope everyone can move forward and find happiness in life.
Love means never having to say, "Put down that meat cleaver!"