Sounds like he's enjoying the challenge of ruining your plans. He is clearly in violation of the parenting plan and therefore, he's opening a can of worms, isn't he?! Add the 'cost' of mediation and it looks like he's self-destructive. You're in a tough spot AGAIN, Muse. Right where narcissists like to put people.
I mediated my divorce which is the only basis of experience I can draw from and yes, the attorney was able to see WHO was resistant to resolution and who wasn't. The narcissist's aggressive instincts rise above their ability to control the situation with pretenses of humility and self-righteousness. I maintained my 'cool' the whole time even though inside I wanted to explode. Just because the attorney 'called' the X on his aggressive and critical behavior, that didn't resolve anything. We had to work things out together. My plan was to have three plans. That way, I wasn't caught off-guard when the X refused to agree to Plan A. The hardest thing is being caught off-guard and not being able to think clearly because our emotions are riding high.
What does he think he'll achieve and do you think he's 'posturing'? threatening mediation (which he knows neither of you can afford) just to get you to back down and prioritize his 'time'?
Hugs,
CZ