I would stay strong, like in your first reply email. Remind him that no matter what the court decides, he needs to make the plans with his kids directly, not through you. Reassure him that you will support the kids in making plans directly with him and sticking to them - BUT THAT IT IS HIS RESPONSIBILITY NOT YOURS to make and keep plans with the kids.
I would just say, sorry if you don't buy it but that's the truth. If he wants to fight in court, don't show you are scared of this. But remind him that the court is not going to babysit your kids and bring them to his house. HE NEEDS TO STOP BLAMING YOU. YOU DID NOT MAKE THEM NOT SEE THEIR DAD.
You may also want to remind him that the kids have a lot of painful feelings about what's going on right now between you two and that he needs to talk to them about those feelings and be open to validating them. You are not the one creating those confused feelings in the kids, so he needs to not blame you.
You can do this. BE firm and repetitive of what he needs to do - take the focus off of yourself and don't defend yourself. this is not about you.
And my father always said, When you don't know what to do, do nothing.
