I hang out a lot at marriage builders.com. They have a huge forum for infidelity, how to recover, etc.
One of the posts on that forum that was so incredible was a woman who'd had an affair with a married man and her regrets, years later after she'd married this guy.
She talked about how the guilt just ate her up. His family never accepted her and his resentment towards her grew as the years went by because they had destroyed his family together.
She never felt like she could trust him and he withdrew from her over time. She talked about how excruciating her marriage was because it all started in such a sick and destructive way.
I get the feeling your ex probably resents this girl. Or he will over time.
And she tries to play perfect mom to show him that she never intended to hurt his children, it was just true love that brought them together.
She desperately wants to make it work, to prove to the world that the havoc they created was worth it.
And every time he starts longing to have his family back (and he does!), she has to deal with that and try to fix it for him.
He can't answer the phone when his hurting child is calling BECAUSE HE AND HIS TRAMP DESTROYED HIS FAMILY.
And now he has to live with it.
She accuses you of spying on her and such nonsense because she lives in a fear-based, guilt-ridden reality and there's no doubt that people do talk about her behind her back and know her as a home wrecking tramp.
Her paranoia is a reflection of her guilt and the knowledge that she participated in a crime of sorts with your ex.
Not that any of this brings comfort...
You've taken the high road with all of this and have much to be proud of.
They have each other and they're going to slowly start tearing each other up as they realize that their 'love' can't sustain under the conditions it was created in.
Shame on them.