-Is there a way to break up a PU I am not a part of?
-Why can you only send missionaries to your own provinces?
1. Yes and no. You cannot just demand it in treaty(from what i know), but you can lower prestige of your enemy, and destroy his army, so at his king death, he will lose his PU.
2. Because it is so designed in game. I know it is quite stiupid, But who can do anything about it?