Your ex just gets me angry. That is putting it mildly.
Yes, I would cut and paste the section of your divorce decree to any response that is going to him.
I would then have the standard paragraph with the indemnification clause from the divorce decree next.
Third paragraph would be that he could file a motion of modification of visitation time (or whatever the matter may be), and if the modification is approved that you will strictly adhere to the modification, as you will now be strictly adhering to the divorce decree now.
No, it is not right that he keeps changing times. If he is to have them Friday pm to Saturday am, and can't make it, that means that you would have to constantly change your plans. That is not reasonable, but that would be my argument to the court.
The only leeway that I would give here is to state, "I am leaving the house at 8am, taking the children with me, if you can be here before that time, then you can have the children for the rest of your visitation time on Saturday. (Only problem with this though is that he may keep them later just to ruin the day.)
If he shows up at your house Saturday pm, he is an unwanted visitor to your house, and you do not have to let him in and you can call the police to have him removed.
BTW, I thought he already moved prior to the final divorce decree, and if he knew he couldn't make it on Friday nights, why didn't he change it then to a more convenient time for him.
Good luck
(((((hugs))))))
amy