So, I was in a somewhat similar situation, although not quite as extreme. I was the cheater. We tried to reconcile, did couples counseling, read all the books, etc, but after 4 years, I realized that he would never be able to move past it. When we first separated, we were both so relieved it was over, the divorce itself was easy and uncontested. 50/50 custody.
We both started dating before the divorce was finalized. I fell head over heels for a new guy and that's when shit started to go sideways. My ex started coming into my house while I wasn't home and making snide comments about little gifts my boyfriend had gotten me. Then he started dating my boyfriend's ex-wife and feeding her all manner of lies about me and my former relationship with him (I had been very transparent with the new guy about my affair, but these were complete fabrications). The ex wife eventually said she would prevent my boyfriend from ever seeing his child again if he continued to date me. So that was the end of that relationship.
And a few months later, my ex husband sabotaged a job opportunity I had by creating an anonymous email account and emailing the employer that had offered me the position with more lies about me. When I confronted him about WHY he would do that, he said he wanted me to move far away. And so I did. The next year I got a job 2 hours away and moved with a new partner I had been dating for 9 months. It fucked me on my custody agreement. He won't allow the kids to transfer schools (even though the schools are objectively much better where I am), so we have more like a 70/30 split now.
For a time I had to refuse even communicating by text with him because he would harass me or ignore messages regarding the kids when he drank, depending on his mood. We only communicated by email and I copied both my mother, his mother, and both of our respective partners on every email. I was so tired of him bullying me. I refused to go alone to any exchange of the kids, often taking a male cousin with me and only meeting in public spots. He hated it. But he didn't pull crazy shit when it was all out in the open.
Things have improved since then, but only after I broke up with my most recent partner. He feels like he "won" because I am alone and now he is being civil and pretending like there was never any problem.
I'm so sorry you're going through this, OP. The only advice I have is to continue doing the next right thing, regardless of your ex's actions. Don't let her bring you down to her level. Take the good when you get it and don't let the bad rock you. Keep firm in your boundaries. Move away if you need to. Out her behavior to her family if you think embarrassment would be a deterrent.