Re: My husband is obsessed with/feels responsible for his ex
I'll add to the chorus. I am divorced and remarried with two of my children being from my first marriage. The vast majority of the people I know are also divorced with children from their previous marriages. While I know a couple of folks who have a friendly relationship with their ex's, inviting them and their SO's over for BBQ and occasional gatherings, most only speak when it is something related to the children's health, safety, or education and the occasional scheduling issue. So, no, you defintiely aren't just being insecure and jealous.
Have you thought about contacting his ex directly, discussing the issue with her, and staging an intervention of sorts? You stated that your H's ex has not asked him for help and has acted skittish around him, clearly indicating she doesn't want the same level of contact he is comfortable with. She could be an ally here.
Also, I'd tell her about her beau being married as gently and as soon as possible. It's the decent thing to do. After that, whatever happens between her and the MM is between her, the MM, and his wife.
Follow the evidence where it leads and question everything.