1) it has the ability to cause the region to switch realms, doesn't it? If a priest declares religious state in a foreign realm, that region joins his realm. This restriction is so that a noble from Realm A doesn't take a region from Realm B and give it to Realm C. Just like Realm A can't TO a Region from Realm B in the name of Realm C. Yes, it is a religious option, but realm affiliation still matters. (Re: arresting priests.) Also, I thought this might help give incentive for religions to be willing to spread their elders out across multiple realms.
2) and 3) are for the same reason that the original proposal had the requirement that the recipient be an elder for 30 days. It's to prevent potential abuses along the line of "hey, Frank's got good oratory skills from being an ambassador, have him switch to priest, we'll make him an elder, then when the TO is done, he can switch back and we'll demote him". I.e it helps prevent gaming the system.
