"...today he has sent my L an email telling her that D13 has said if it wasn't for his partner she would go live with him."
Spooky how well you were able to predict what he would do with that. Still, I don't see how it will help him-- nobody is gonna believe he'd actually be willing to sacrifice his "partner" for custody of your D. He largely ignores her and her older sister and favors your S, right?
Consult your L and consider all the the angles before you take action. I too suspect he is deliberately pushing your buttons in hopes you'll do all the dirty D work for him and then be forced to sell the house.
