D.J.
I want to offer 1 suggestion and say THANK YOU for something you shared.
With difficult ex's - I have found that mediating together in the same room wasted all day and all the money because of the exact same dynamics as you noted. However...a mediator WILL allow you to be in a separate room, your ex in a separate room, and the mediator will go back and forth between rooms. Had I known this was an option earlier, I would have required it. I will NEVER meet in the same room with him again, ever, for mediation. In my case, I can't even do conferences or meetings with my child's counselor together. When my ex couldn't get to me voluntarily comply with his expectations, he resorts to manipulation, followed by door slamming in front of my son's counselor, then threats and then using my son as a weapon. I can only feel safe to speak up when he is not there to interfer with that process for me. My ex is a Narcassist and that fact won't ever change so I had to change how I interacted.
Second - thank you for how you phrased your sentence "He's always played the fact that he thinks I'll do what's best for the kids - as long as its convenient for him". I have never had a simple way of explaining the dynamic in my case, until your sentence. Thank you!
Good luck!
D.