While no one would ever say I was physically abused, the verbal abuse was horrific. It has taken me years to get those tapes out of my head.
I agree abuse doesn't have to leave a physical mark. There are all types.
While I do not know your history, it seems to me unless, it is an official document from the court stating a change in visitation, the certified letter is useless. You could reply that when he gets a law degree and becomes a judge you might listen, lol.
You know what is best for your children. Sounds like he doesn't but wants to execute his rights without going through the proper channels.
I hope your children have a great summer whether they see Dad or not.