There is no doubt that when it comes to seating issues etc. the responsibility lies with the ground staff whose job it is to ensure all pax are boarded, seated, baggage on board, inappropriate items produced as cabin baggage are tagged and place in the hold. Catering is also a ground staff responsibility. Engineers will carry out refuelling and ensures the aircraft is serviceable, any snags still carried are within the Minimum Equipment List for dispatch etc. The ground staff will sign their section of the load sheet, which is passed to the captain for counter signature, same for the engineers, they will sign the technical log confirming the aircraft is fit to fly and it is then countersigned by the captain. Now the captain has accepted the aircraft as his/her responsibility on the basis that all known, or acceptable as 'fit to fly' issues, have been resolved, there is nothing outstanding except to close the doors, both these documents are legal documents when properly signed by all parties..
There really are no ifs or buts about this with airlines of any size. Airline operations manuals, traffic manuals and engineering policy manuals will state the duties and responsibilities of the staff in their respective departments and it is the regulators job to make sure they are adhered to by the airline.