So it begs the question - when does an 'instruction' given by the CC or TC become a 'lawful command'
Nowhere in JQ's conditions of carriage (currently dated April 2022 on their website) does it state that a passenger must sit in their assigned seat. The words assign, assigned, or assignment aren't even in it in relation to anything. Seat Allocation is a section heading only followed by fluff.
Now a boarding pass would constitute an 'instruction'..
JQ don't define "Boarding Pass" in their Conditions of Carriage. QF do though and it's
the document (in paper or electronic form) that is issued to you as evidence that you have checked-in for a flight. Hardly an instruction. QF also don't mandate sitting in assigned seat which is why JQ's lack of same is understandable. VA do not mandate it either. In fact, almost all references to seat or seating are there for the airlines benefit as to why they don't have to take any notice of any preferences or whether you've paid for a particular seat class or whatever.
I looked up some random airlines. Most had nothing, but I found this in United's Contract of Carriage:
UA also prohibits Passengers from selling their seat assignments at any time and/or exchanging them at the time of boarding without first advising a member of the crew. You could argue this doesn't mean you have to get their permission or approval, you just have to let them know, You'll probably end up riding the lightning arguing the semantics of that though.
While JQ may have an expectation that a passenger will sit in their assigned/allocated whatever seat, they haven't stipulated it as a requirement of carriage to do so.