A law system would have to make sense and be lax enough to reflect post-apocalyptic rules. There would be more witch-hunts, and rigged trials based on your reputation. I liked the idea of the contract killers and regulators but they could I guess have more authority than just being surprise encounters too,
However it would be frustrating if the system was like Oblivion's where you cant have any fun without a guard finding you on satellite and forcing you to confess or fight.
No. The law system should be flexable with relativity. Highly guarded military areas or higher class settlements should have some sort of penalty but the villages and small towns should have smaller power to control you.
Punishments should be based on the level of your offenses and type of crime a long side with multiples of said offense while depending on the area. Only the cities like Megaton would ideally jail you, raiders would tie you up and torture you, the military would kill you or experiment on you like terrorist or treason crimes, and wanderers would just fight you.