Wow, good job using your call list and taking care of yourself!
X is a selfish, fearful, petty creep. It really was not fair to the kids for them to be the ones to let you know. How about:
"X, for the kids sake, in the future, please communicate major life changes that affect the kids' lives to me directly so that the kids are not placed in the unfair position of being the ones who have to inform me. They don't need to be in the middle. Email can work great for this. I don't need big explanations or justifications. Just a simple and direct note would make a big difference in terms of taking the kids out of a role they shouldn't be playing. Something like: "Donna, this is just a heads up to let you know that I and GF will be buying a house together and we are taking all the kids househunting this weekend." Thanks! Donna"
As for X, he chose to effectively live with his GF long ago. Now they are simply getting more reasonable housing. This is not something he is doing to you. It isn't about you. Look at it this way, at least a house will be a better environment for the kids than a cramped apartment.
In a few days, it might be worth reconsidering S's request to change the visitation schedule. I totally understand why it is something that you can't look at today. But once you get more level, try to ask yourself and maybe the kids' counselor: What is best for the kids? I really have no idea, but there must be some research on it. For me, I imagine that fewer switches would be better.
