You can have many schedule plans as you like. But come the day if the person is not up to the job due to illness or personal reasons you must have plan B or a standby. I believe Smith was . Just remember Smith was an human and not a ROBOT!
He has his other officers. In case of illness at sea the Chief Officer would be his partly replacement. On land there would have been others who could take over as Captain Bartlett.
What is your reason to believe Captain Smith was "under huge pressure and stress and clearly lacking in shore time leave"? There is no single evidence to this. Captain Smith had also the time to go home to spend the night there. This does not sound for someone who is under big stress and pressure.
Sorry Mike but you are posting only one believe/theory after the next and even seem not to take time to give a repsonse to questions asked to you. All answers we got is that you did not claim the coal fire was the reason for the sinking or that it is not what you claim but what you got from the book by Molony.