you might agree that compulsorily adding a CC to the Flt Deck whenever 1 pilot remains, "as a protection against mental illness", might not appear the wisest mitigation
The reason for adding the CC is to be able to open the door if for some reason that is necessary. Or to put it another way, so that no single person will ever control the door lock. CC is is not there to evaluate the actions or mental status of the remaining pilot, clearly they are totally unqualified to do that.