Golden horde concerted to orthodox and then catholic ...also questions on succession crisis

I had this happen in my Wales game and have a few questions on that and also would love help on succession crisis.

First, could the golden horde join the hre? Is it possible to have a khan the hre?

On succession...

I control Wales and half of france(for some reason England wanted to be an ally so I let him) and now every time there is a succession I have a full blown civil war. A majority of my vassels end up in prison and France manages to take 2-3 counties away during the crisis. It has gone from a challenge to outright frustration. Oftentimes the revolting vassels have 30+ relationship with my heir and lack the ambitious trait. I could use advice on how to avoid fighting half my vassels every 20-30 years. Btw succession laws are primogenture and there is medium crown authority.