I wish I knew the words that would make some of this go away my friend. Your wife has become someone that I could never imagine anyone becoming.
I do believe it's time for you to flex your muscles, even if just a tad. You have done it in the past with some success (thinking of the short shorts).
Is it not possible to forward to her lawywer through yours a more "formal" informal agreement on some basic time for Mom and Dad only? Clearly there was a verbal agreement at some point in time between the two of you. Now, for whatever reasons and for whatever motivation, she is choosing to directly attack that on a regular basis. I would be inclined to ask my attorney to mail hers with a letter to the effect that X day will be Dad's day and Y day will be Mom's day, and the two of you will attempt to cooperate on the other days.
Not that she would be inclined to either accept or follow such a thing. Seems to me however that it wouldn't hurt in any event to have an official record of you ATTEMPTING to work out a reasonable co-parenting arrangement, even if she refuses.
Short of that, I say stand up for your informal time with the kids. Keep it short and simple. Tell her that the plans made for your day with the kids are not doable, as you have previous plans and intend to keep them intact.
Yes, it is possible that she will make a scene. You cannot control her actions, neither can you completely insulate your children from their mothers irrationality.
You would not let a burglar come in to your home and not challenge him simply because you did not want your children to see confrontation. I know, it's a bad analogy. But the point is that such unreasonable actions will always continue if no one steps up to put a stop to them. I believe that there is a well known quote to the effect that evil can only flourish when there is no good man of courage who chooses to stand up to it.
You have spared your children much and in the process taken it on the chin time after time. Each of us would do much the same. Still, there comes a time when we finally have to say "no more." If anyone can pull it off, I know you can.
Sorry I've not written much. Quite honestly, I am at a loss for words (printable ones at least) when it comes to your wife's actions.
Blessings,
Bill
"Don't tell me the sky is the limit when there are footprints on the moon."