I completely agree with the avice Another Stander gave you. Remember, don't set boundaries to things you can't do anything about. Many H's have set boundaries about OM being around the kids.....but unless you can show a reason they aren't safe and get a R.O., I don't think there is much you can do about him being there. It stinks, but that is the way it is.
But if you set that as a boundary, what action can you take to protect yourself? About the only thing left to do.....and save face, is to follow Another Stander's suggestions. Otherwise, prepare to become a punching bag for both WW & OM!
You have to get a lawyer and get legal suport to have your kids. Don't talk to OM, and don't take his and WW's threats and assault. Don't discuss anything with her face to face, since that incident. Prefably an email would be better, IMHO. But, you don't have take her physical assault anymore. If you have to get the police involved, so be it.
If her parents will intervene and help transfer the children between you and WW, by meeting you some place away from the house and without the WW (and possibly the OM) included, it would protect you and the children from those types of scenes.
So sorry you are going through this mess. ((hugs))
Please keep us posted.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!