Sorry you had such a tough weekend. To be honest, it sounds like both of you spent the weekend sticking your tongues out at each other, lol. But, on the bright side, you did work through it without a major battle :-)
With respect to D staying over, I certainly think it was appropriate for you to ask if OW would be there. Maybe when he asked why, you could have given him the complete direct answer you posted here: "because the two of us need to talk to D together to prepare her if something like that is going to happen." I'm not sure why you feel as though you shouldn't ask about such a thing with your D involved. What you want to avoid is a situation in which he is sneaking around introducing D and OW...
Now, my real point... I'm not sure how much that discussion had much to do with D at all. I think it was about you trying to ask for reassurance that OW is out of the picture and SO trying to give you that reassurance.
If that rings true, then maybe it is time to simply do this directly. "SO, a few times I've felt you are trying to tell me that things over over between you and OW. I'm not sure, though, that is what you meant or what I want to hear."
