Most religions are not going to conduct background checks on people interested in becoming full members, nor should they have to. That's an onerous burden to place on them. A character doesn't have to tell you he wants to be priest, he just has to become a full member, then can become a priest at any time with or without prior notice.
Religions aren't secret societies; they should not need to screen every prospective full member to make absolutely sure they can be trusted. Religions are by their very nature inclusive, and have to be in order to thrive. Your continued intimations that they should be more careful and less trusting run contrary to what a religion is supposed to be, both in terms of game mechanics and in terms of what a religion is in real life.
I am sympathetic to what you're saying about coding limitations. If the Elders vote approach won't work, what about black marks, they way a lord can kick out a knight? Maybe reserve it for the ranking Elder? Or hell, honestly, why not just let them be kicked like any other member? As you've said, the complexities of whether or not he'd even want to take followers with him are simply beyond what we can do at the moment. Instead of idealizing that scenario, why not just fix it right now by giving religions the ability to kick them out as any other member, then address the entire issue with as much depth and detail as desired at such a time as schisms can be fully implemented? The current situation is not good. I get that you'd like kicking a priest to be a richer source of conflict, but we just don't have the mechanics to support that now. Leaving things as they are is not better.
