1. What do you mean by prophet-status? You won't lose your founder position. But a religion must maintain half of its elders as priests, or all the temples lose one size every day.
2. Religions spread into other regions, especially regions where they are no temples to maintain beliefs.
3. No. As far as I know, the only way to ever take over a rogue region is militarily with an army using traditional takeover methods. There is no buying, religious claiming, or other means to acquire rogue regions.
