Space shuttle had an empty weight of 170,000 pounds and a max landing weight of 220,000 pounds. The lunar lander was about 35,000 pounds, so you’d have 15,000 pounds of spare capacity for landing. Presumably “empty weight” means entirely devoid of fuel (e.g. for the OMS), astronauts, sandwiches and beer. I’m not sure how much of that 15,000 pounds would get taken up by those things, but it seems like it might be close.
You’re comparing it to “private vessels” under maritime law. Under maritime law, privately-owned vessels may be deemed abandoned by the original owner. Government vessels are never abandoned unless the government expressly forfeits all property rights. Without an overt declaration of abandonment, the government owns all of its shipwrecks and property at sea. See United States of America v. Richard Steinmetz, Appellant, 973 F.2d 212 (3d Cir. 1992) for an example of the US Government demanding the return of property salvaged from a Confederate shipwreck. The United States gained all Confederate military property after the surrender. So all Confederate shipwrecks belong to the United States government forever, unless and until they state otherwise.
Article VIII of the Outer Space Treaty has no clause for abandonment. Governments retain the property rights of everything they launch into space. If another country damages or steals jettisoned Apollo stages, the United States can petition for the return of the item and/or compensation for damages.
As for private spacecraft, Article VI of the OST states that countries maintain control and jurisdiction over private spacecraft launched under their registry. So, International Law says that the United States has authority over anything launched by SpaceX (and registered with the United States). Under that authority, property rights are disseminated according to whatever property rights that government has. So, SpaceX could not appeal to the international community over property rights. Property rights belong to the country. It’s only because the US recognizes private ownership rights that Musk maintains ownership of the Tesla that he launched into solar orbit!