I should think that CC are sufficiently qualified to open the door to let the other pilot back in
Please confirm that you would "empower" the CC in this scenario to override the specific verbal instructions of the Captain?
If that had been done on this flight, then the outcome may have been different
It almost certainly would have, but then it would not have been planned in this way. You cannot look at one accident in isolation - you need to design a "system" to provide the minimum overall risk.
"Mentally ill pilot crashes aircraft and kills all on board". How best to address? Prevent the situation in the 1st place, or accept it as fact, and devise solutions that not only allow the mentally ill pilot to still achieve their aims, but opens up a whole range of new hazards.
PS I assume you do know the rules (or lack of) re CC operating with depression / mental illness / associated drugs, their medical checks (nil) etc.?