One-voter/posters are still active and have to be to avoid being modkiled/replaced. Come on McFly, that means they have a reason to be alive and thus are doing things in the shadows. Mafia and rookie cops do that. (a real cop would keep the discussion flowing and find some suspicions to work on)
Having a purpose and fulfilling it are not the same thing. I fail to see why someone's activity should be linked to their alignment. And going by your logic, why risk a cop, even if they are a rookie?

Not everyone, just town. Mafia doesn't have to kill themselves. If you have two inactives and one's a mob, do you get him on the first try? Or you down two town (remember night kill) before you get your man? What if there are more inactives? I gotta read the rules, but I hope our mods don't allow inactives to idle.
I'm not sure what you mean.