I think this is a good proposal. As for the rewards for reporting, perhaps the fixed amounts should be somewhat lower, because the 5% share of recovered funds is a good enough incentive in my opinion.
Also, the fixed amounts for rewards should be denominated in USD (but paid out in ARB), just in case the ARB price goes up significantly.
It would also make sense to have a pilot phase for the program (measured in months or until funds run out - whichever is first), and evaluate everything after it (including the platform), so that it can be properly improved upon.