I saw my IC again yesterday. On the subject of the holidays he said what I already know -- don't let W get away with trying to claim all of the important days coming up.
On the subject of my W and her nephew's (W's sister's son's) wedding, he (my IC) also strongly suggests that when W announces her plans to go take our S's to Mississippi that weekend for this event that I tell her I will go as well. I should not agree to W taking our S's on a weekend (my agreed-upon normal time to be with them) unless I get to go as well; I don't necessarily have to be invited to the wedding either, but to be there to visit with my family and take care of my S's. That way I can ensure that my S's get to see their grandmother (my mom).
I am giving this some serious thought and am inclined to do just that. On the other hand, I also know that this will likely be seen by my W as an act of war.