To be perfectly honest, I think that you need to butt out of all the arrangements. Let him deal with his Ex and his mother the way he wants to. Why do you want to be involved in all this palaver?
His arrangements with his ex wife are exactly that - HIS arrangements. You may not agree with them but you need to accept that this is how he wants to do it and to stop being so controlling. He is right and you are wrong.
You're focusing too much on you and your feelings in all of this. Why don't you just create a nice life for yourself and your husband and let him do what he wants with his kids. Stop competing with the Ex. Stop allowing her to control your responses. Stop taking the MIL's comments to heart. Accept that you can never compete with the Ex and his kids as far as MIL is concerned.
You can be in control of your attitude, and if you don't back down you'll drive a wedge between yourself and your husband.
In the end adherence to the court order is not important. It's how you respond that's important. The more you push and complain about not having his kids the more the Ex will resist. I would suggest that you all back off, believe me I know from experience, once you feign disinterest and the ex wants some time to herself the kids will be over at your place in the blink of an eyelid!
Chill. Is it worth breaking your marriage over?