One last word on this. I think you've got to choose to focus on theme or game function. If you really want to do a theme, you can't leave characters out for the sake of fake claims. Even if you have a good theme, mafia can always claim civ. If you feel like that tilts the balance against mafia, give mafia more powers or more information, so they can figure out claims that won't detract from the theme. Or just say that people can't reveal their characters.
I've got a great game that I'll put on the calendar at some point that addresses these issues. Nobody should take this as personal criticism because running a game is a pain in the ass and I respect everyone's creativity and effort. But I think if a game based on an established book, show, film, etc should to stick with the source material as best possible.