3rd person as a suicide safety measure is meaningless
Question: What could a Cabin Crew person perched on a jump seat do to prevent a suicidal Pilot closing the thrust levers, stuffing the nose down and shutting down the engines as the a/c starts a high speed dive?
Or preventing matey from shutting off the fire valves in an action that takes less than 3 seconds?
Answer: nothing.
In the Egyptair 990 case, the Captain was unable to prevent the demented FO from killing all on board despite returning to the controls. Ditto Moroccan, ditto Silkair, ditto JAL crash.
Sadly a determined and qualified suicidee can achieve their goal despite attempted intervention. The most publicised case was United 93, where the Arab hijack team crashed the aircraft despite eventually being overpowered.