For me, a PVE mode, should not allow friendly fire, or ramming. If all you want is open with a gentleman's agreement, why bother? That's what groups are for.
As for the "rules" you recommend, how can someone pay "the debt" if they can't undock to earn money?
While I hate to use the word immersion, I do not think magic shields are the answer.
If the person breaking the rules gets booted permanently, the victim gets fully reimbursed it would be fine.
As for paying the debt that's easy, sell the current ship and buy a sidey if they have to. Might sound a little harsh but if people deliberately break the rules after agreeing to them, tough luck. If you can't do the time, don't do the crime. Also I doubt it would happen often after the first few "I quit FD sucks for enforcing the rules" threads.
Someone checking a box saying they have read and understand the rules and punishments that FD will enforce is not really a gentleman's agreement, more like an EULA.
As to why bother, I use all modes and have Mobius for PVE, >90% of the games owners are not even registered on the forum, why should they not have the same option to play PVE if they want to, the only way to reach these people is via the game launcher.