If the (sunk-cost) common insurance pool pays for it, where's the incentive not to pollute? We're right back where we started with a tragedy of the commons.

For the proper incentive structure, you have to treat the money the way we treat all proceeds of crime - it's forfeit. If you buy a stolen bike, you don't get to keep it when the police come knocking. Ignorance of the crime doesn't earn you the item - it simply means you don't go to jail.