Excellent synopsis of the situation, welcome to the board.
Unfortunately you are dealing with a full blown cult, as you probably realize. You have figured it out, you pretty much can be the go-between because you haven't dedicated yourself to the cult. Contact will always be limited and strained unless your fiance wants to play their silly game and get reinstated. Unfortunately it would probably take a few years of attending meetings and being shunned before he would be reinstated.
On the plus side, he could flip the middle finger to the whole organization if he never attended another meeting after reinstatement and his family then could talk to him.