Welcome aboard. Just to cut to the chase....I would suggest that you do NOT attend his family get-together. If you want to visit them when he's not there, find. I could give you my reasons for saying that, but to shorten it...let me just say that it is "his" family and no matter how supportive they are to you right now...you do not need to be at his family's events when the two of you are S. It would compound problems.
I agree that you never give empty threats b/c they don't work and they can backfire really bad. You can't keep your H from seeing his children, unless you live in a country that has different laws than most. And....why would you even tell him that if he moves out on his own that he could not see his children? Had you rather he live with his parents?
Here's the thing sweetheart, I see what seems to be a lot of controlling on your part. That is what must change. Nothing.....and I mean NOTHING will work for this M until you stop trying to control him. When he did not respond to your techniques (no offense) then you tried going through OW and when that didn't work then you are thinking of going through his family. Now you are threatening him b/c you don't want him on his own. That does not work and it could be one reason he wants out of this R. Just a thought.
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!