Wow, who knew that the water was that shallow five miles out to sea? The guns that were placed in Sumter in 1897 were 12" M1895s, with a range of 18,400 yards.
My recollection is that the entire entrance to Charleston Harbor was very shallow, and that only shallow draught ships could navigate it. Was it shallow out to five miles? I don't know, but I could find out if I really wanted to know, but it is beside the point. Why would a WWI era capital ship try to come up Charleston's harbor? And if it did, would two guns stop it? The Geography of Charleston harbor was a sufficient defense all of it's own.
"Fort Sumter was designed to mount 135 guns and provide living space for 650 officers and soldiers. "
650 men? That's what it was designed to house, and when did it ever have such a garrison?
Never. Just two coastal guns after 1897, and which never saw combat. Pointless. Big waste of money. Never amounted to anything useful.