In 1805 the State of SC ceded the property to the federal government with the stipulation that the property would revert to SC if the following conditions were not met:
existing fort and/or new forts had to be built (completed) within three years
fully garrisoned at all times/never abandoned
If either of these conditions were not met the properties in question reverted back to the state. This 1805 cession was referenced as applicable in the cession of the shoal were Fort Sumpter was built.
The federal government failed to abide by the terms of the cession, the properties in question legally belonged to the State of South Carolina.
I've heard of the legislation you mention but I've never seen it. I have problems with it since it seems to fly in the face of the Constitution which says that the government will have exculsive legislation over property obtained with the consent of state legislatures for the purpose of building forts, magazines, arsenals, etc. Your 1805 law indicates that it was done with the consent of legislature. And regardless it would seem that any earlier laws would be superceded by the act of the South Carolina legislature passed in 1836:
COMMITTEE ON FEDERAL RELATIONS
In the House of Representatives, December 31st, 1836
The Committee on Federal relations, to which was referred the Governor’s message, relating to the site of Fort Sumter, in the harbour of Charleston, and the report of the Committee on Federal Relations from the Senate on the same subject, beg leave to Report by Resolution:
Resolved, That this state do cede to the United States, all the right, title and claim of South Carolina to the site of Fort Sumter and the requisite quantity of adjacent territory, Provided, That all processes, civil and criminal issued under the authority of this State, or any officer thereof, shall and may be served and executed upon the same, and any person there being who may be implicated by law; and that the said land, site and structures enumerated, shall be forever exempt from liability to pay any tax to this state.
Also resolved: That the State shall extinguish the claim, if any valid claim there be, of any individuals under the authority of this State, to the land hereby ceded.
Also resolved: That the Attorney-General be instructed to investigate the claims of Wm. Laval and others to the site of Fort Sumter, and adjacent land contiguous thereto; and if he shall be of the opinion that these parties have a legal title to the said land, that Generals Hamilton and Hayne and James L. Pringle, Thomas Bennett and Ker. Boyce, Esquires, be appointed Commissioners on behalf of the State, to appraise the value thereof. If the Attorney-General should be of the opinion that the said title is not legal and valid, that he proceed by seire facius of other proper legal proceedings to have the same avoided; and that the Attorney-General and the said Commissioners report to the Legislature at its next session.
Resolved, That this House to agree. Ordered that it be sent to the Senate for concurrence. By order of the House:
T. W. GLOVER, C. H. R.;
IN SENATE, December 21st, 1836
Resolved, that the Senate do concur. Ordered that it be returned to the House of Representatives, By order: JACOB WARLY, C. S.