It has always been my understanding that total command, from a legal point of view, commences when engineers and traffic staff have completed their tasks, the load sheet is signed and the doors are closed. Obviously the captain will have important decisions to make from the time he signs on until the doors close regarding the operation of the aircraft and crew but it is unlikely he will become involved in seating disputes, other than if jump seats are requested. The dispute here is entirely the responsibility of the ground staff and does not come within the captain's remit.