He had the "authority" to do anything he D@mn well pleased, but Washington DC and his chain of command did not place him in Sumter. *HE* placed himself in Sumter.
His decision was a tactical decision to provide a more easily defended position then the open pit batteries at Fort Moultrie.
It wasn't defensible at all. It was an illusion that a military outpost can exist among any people without being backed up by a huge force that can enforce it. It was rather wrong headed thinking, to believe you could hold such a position against the forces that were likely to be brought against you if you antagonized them.
Lincoln's generals told him before he sent his war fleet that it would take a force of 20,000 men to take and hold that fort, and it was simply not worth it.
Also his war fleet would have been sunk had they carried out their orders.
It was more defensible than Moultrie.
“Also his war fleet would have been sunk had they carried out their orders.”
His “war fleet” carried out their orders. Since they were not fired on by the Charleston batteries, they did not fire on them.