If the GW accident scenario is correct (which I think it is), the solution is not to allow suicidal pilots on the Flt Deck, and mitigate that with an FA. The solution is to avoid, somehow, those pilots whilst in that state of mind, being on the Flt Deck surely? (BEA point 2)
My personal experience is that most suicidal persons prefer to be alone, or at least feel alone at the time. An "audience of one" even if it were a case of going out in a blaze of revengeful anger and glory. So the FA in the cockpit could well be at least a hinderance in general; albeit of unknown specific usefulness in this case.