Consensus?
So after 130+ pages of this thread, there would appear to be consensus that no procedural modification would have prevented this tragedy, bar the presence of a third crew member. The only obvious solution would be to enlarge the FD to include a toilet. But even that cannot offer total protection against someone bent on carnage, for reasons already noted.
Conclusion: the statistical risk of a recurrence remains minuscule. Regulating authorities and companies must swallow hard, and not allow paranoia and the frenzy of the moment to force it into inadequately considered responses.