That armoured lockable door is the real threat, as we have seen on several suicide flights and at least one where pilots were incapacitated.
Spot on, Mach, and that, amongst other reasons is why the extra person is there.
Where I once worked, if wasn't necessarily a flightie but often paxxing tech crew who came in when an operating pilot needed a loo stop. It was the practice to make sure a flightie knew the reason they were there and they were also told to sit on the jump seat and, if necessary, strap in.That overcame the concerns DSJ has raised.
As an exercise, ask the cabin crew what they think the purpose of them being on the flight deck whilst the other pilot uses the bathroom is. You may be surprised to find a variety of answers.
See previous paragraph, if the tech crew aren't doing that (making them aware) then they are remiss.