I hope your back feels better soon! Did you do something to hurt it?
I think Dan is about appearances, by spending his time with the kids at your house, he can let it "appear" that things are "normal". I think he is ashamed (though not enough to have let it stop him) of what he has done, and the less he has to be public (taking the kids to his parents' house), the better he likes it. Of course, BobbiJo, you know that you are going to have to raise a boundary on that one. What he is doing is confusing to the kids, I don't see any way around that.
I hope things go smoothly for you this evening... I know it's not what you wanted, and it will be hard, in a way. But I really don't see another path that makes any sense at all. HUGS!