There's also being arrested for not wearing a collar which gets you into the prison pretty quickly!
With regards to getting out again, there's also two exits from the "courtroom": one using the ladders through the private quarters which drop down next to Mona's tent, and another in a turret behind some crates which will drop you down into the swamp.
Don't know about the other two, but I wouldn't be surprised if there's other methods...
