Mind control cap is currently bugged. If you mind control a target that is mounted and order your new pet to attack someone, he will not do anything and just stay mounted. It’s really annoying and irritating because it completely changes the outcome of a fight sometimes.
I dont think that is a bug. You cannot cast while mounted. Have you tried forcing the controled player to dismount?
You can’t dismount them. This happens with standard priest MC, too. I’m not sure if this is Vanilla-like or not, as I didn’t play priest then.