I think there are two different subjects here: General or Particular Average and the Salvage Contract and subsequent security. Under the Lloyd's Form the master (or the person entering in to the LOF) signs on behalf of the ship, cargo, bunkers, stores and freight at risk (if any). Though there are occasions when the ship wants one salvor and the cargo another. In the present case there maybe several salvors, either joint or with some acting as sub contractors.,
Presumably the ship was beached by the French tugs as she was in imminent danger of sinking; whether she was beached in the best place will be decided later. I don't know what contract was agreed by the French, or for that matter Smit, or whoever is to remove the oil and the cargo.
It would appear that the ship is in danger of becoming a CTL and, if it does become one, the P&I Club will have to pick up the tab. The salvors can earn their remuneration in two ways: saving property and preventing, or reducing, pollution.
Regarding the Receiver of Wreck, when I first came into the industry (when Noah was a lad) the Receiver of Wreck was a local customs officer and he, and in those days it generally was a he, often went to the site to minimise losses due to wrecking.
Apologies for any typos - my eyes are dim!