You really don't need to engage with him. Believe me I know it is difficult. The more you get drawn in the worse you'll feel, and round it goes.
You have arrangements in place. There was no suggestion that these would be variable. You have agreed to mediation (against your better judgement) to discuss further issues. That is where he can take proposals, and you can respond.
He will kick off. He wants a response. Don't give him one. Reply as above, and say that you wont be discussing this any more via email. And do that. Otherwise he's right back inside your head, and your anxiety levels will ramp up. It is that thing again. Control. You take it back. My late DM used to say this to me when I left DH, and the world was spiralling , and he was never stopping with the demands etc . It annoyed me (I am probably annoying you!). But .... it worked, even if only in the short term, and helped me stay calm.
I'm not suggesting that you and he may come to other arrangements at some point, or that the DC wont spend holiday time with him. But, he doesn't get to tell all of you when and how this will happen. It is about them, and stability, and setting into a new routine, and making new friends, and going out to play are crucial And the summer holidays are the time for all of that. They need this time at home with you, whilst seeing him regularly of course.