"What exactly would the hostie do if the naughty pilot decides to divert? Take over control? Overpower the pilot and force him/her to behave? Threaten the pilot with the fire axe? Open the door to allow access to the other pilot?"
Yes, that
"Anyone who flies will know that they can do what they wish with regard to single-pilot operation. Thanks to the locked door policy, this is easy."
You seem confused