Well crazy a$$ ex is a place I can help most. Literally get everything in writing.Every interaction. Anything that can be put into a legal and/or notarized document the better. Reach out to his mother yourself. Go out of your way to tell her you have no intention of ending any relationship between her and the children, and that you'd never want her to worry about that. (I know H hasn't said this to her yet, but he will, trust me) Then make sure you tell her what H said she said and try to give her a small outline of your side of things. And tell her your very sorry if she now feels in the middle but you felt it was important for her to know the things her son is saying and doing, including putting words in her mouth. Because dollars to donuts he was lying through his teeth about that whole thing. You are no longer in save this marriage mode you are in save yourself mode. Do every thing you can to protect yourself and your reputation with those close to you. He will do everything he can to make you look like poo. My exH literally told people he kicked me out because I had cheated. It had been 4 years since I ended the affair. Ignoring literally everything he did in the years following or prior including his AODA and mental health issues. The real reason I packed my things and left.
Next also dollars to doughnuts that EA isn't just an EA. Especially if she's sending the kids gifts. But I'm not 100% on that those gifts are actually from her. If she's still maintaining the facade of her family I'd bet H got those gifts and said they were from her to prime the kids for interactions with her and to p!ss you off. But I'm also starting to wonder if this really was an EA if your H has this built up way more in his head than she does like my H did. They were riding off into the sunset in his head. She was just filling her time. And I was the only one who could see it.
Do not send thank you notes.
Lastly, I second May. You can't put the weight of this on OW. Should she know better? Of course. But you were his wife. Those are his kids. These have been his choices. She was there, but he just as easily could've walked away 1,000 times but didn't
