One of the few defences against insane acts is a proper staff welfare monitoring program.
Very much agreed, but that would cost.
Tell me, how would having a sweet twenty-something or other cabin crew member sitting in the otherwise vacant F/O seat stop me from doing something I really ought not to? Will she whack me with fire axe?
There's the problem and I agree very much with your final point. I see no benefit and actually some risk in having someone sit behind a solo pilot on the flight deck.... there's no point unless the airlines are prepared to engage in something along the lines of (by modern standards) very invasive Positive Vetting for
all crew members.
....worms, can of, open...