I believe Tina was there as a representation of power, especially considering the speech. She was their key to massive power, even moreso than the Magitek Knights.

As far as Ghestahl not wanting a maniac running around, he just could have had him killed. I think it was more likely that he saw a maniac as useful, one he could use and one who was dedicated to the Empire and the quest for power. I think a lot of people in the Empire would've balked at some of the worse things the Emporer wanted them to do, but Kefka not only did them, he did them with pleasure.