But they still leave the biggest problem unanswered - you are trying to "mitigate" having pilots on board who will deliberately crash the aircraft. That to me is an unacceptable premise, and not possible to comprehensively mitigate against. So I think the basic issue needs to be addressed, not accepted?
There may be another way. Most agree that drone/remote control/pilotless is not the way to go at least for the foreseeable. The PF could crash deliberately in close ground proximity by sudden gross control action impossible for PNF to overcome let alone a FA. In this case the pilot chose to set an un-cleared course, speed and descent towards high terrain. It would be fairly easy for an algorithm either onboard, ground or interactive ground/flight to take or prevent contol action in extreme circumstances. It would require a huge amount of technical evaluation and pilot input to find an appropriate balance but I think it could be done and implemented in a useful timescale.