1: He could use his mind control powers, but you would need permission from the other players before using it. A permissions post would work fine for this.

2: You can do that if you like, but it will count as two of your character spots. Additionally, you would have to either do two apps (one for each of them), or one app that covers both in enough detail that we can make a judgement on both personalities as separate character. Either way would be acceptable for apping both at the same time.