Re: Head of State, customary lands
I’m not sure what “assumption” you refer to.
The Constitution is framed around an assumption that everyone involved is filling their role in accord with the Constitution.
It appears that some persons did not perform their duties to the standard of honesty required by the Constitution with the result that the former Head of State could not rely on advice given from those persons.
But the fact remains that the Head of State was required by the Constitution to sign the document placed in front of him by those persons.
To answer your question - indeed it does not make sense to have a Head of State - unless all other officers of Government and Parliament are performing their duties with honesty and integrity.