You won't be able to convert people outside your counties, since your neighbours will be mostly Miaphysite Christians and Muslims (you might have had some success if they were pagan).
As for "how can I avoid being invaded and losing" - that's not the sort of thing that can be given in a Quick Answer. You'd have better results starting a thread on the subject - I'm sure someone can give you some advice about it - or searching the forum for previous threads about Semien.
Yes, and to further highlight that coversion probability is highly related to MA. So if your MA is below 80, it tends to take a pretty long time. And the only way to boost MA is to claim back at least 3 or more religious sites.