I don't know if it will help any, but I'm in the exact same part of things as you are with DD meeting OW and going to X's "new house" without my support.
On the D/S forum I posted some stuff this morning because I had to work very fast this week to get a grip on myself, for I fell apart again as this came to light all at once.
Yes, we the BS are the common enemy, for many reasons. One thing that I think lately is that we represent reality and our WS's often can't stand to face that.
And by bringing our kids into their lala land, they can try to grow the seed they sowed and make it more real than it really is. Kids are people that can be manipulated for a time and can help them think it's real or justify, but I think kids are very smart and will see through after some of their own confusion dies.
I'm cycling some of the same emotion that you are but my realization that helps lately is that it's not about me..
It's about our WW's trying to collect people, like I said and the poor kids don't know which end is up for a time so they will fall for the glitter at first.
DD here is so revved up that I wear earplugs now when they have their phone talk or when he comes to get her and she fires the questions at him. It's such a massive trigger, because it's my daughter too and his regard for me is zero and blatantly so.
That was the hard part for me, that he wouldn't wait for my heart and mind to catch up, but he's always been about control.
I wish you well and though I have little bouts of tears left, I am getting better when it comes to mind.
I'm also wondering about background checks for the people she will be with, including X because he's so different I don't know him anymore.
A person is a person, no matter how small. -Dr. Suess