Wrong, wrong and wrong. It was April 12, while Sumpter was being shelled, not the day before. It was the USRC Harriet Lane, (U S Revenue Cutter) that fired a shot across the bow (they did not fire for effect) on the Nashville because the Nashville had no flag, US or CSA, on her mast. After the shot, the Nashville got the message and raised the Stars and Stripes and was allowed to pass. A US Cutter would have fired a shot across the bow of any ship entering a US Harbor that had no flag be it in 1861, 1821, or 2001. That is the protocol.
Not true.
"Designed as a revenue cutter, Harriet Lane was named for President James Buchanan's niece, who served as the bachelor president's unofficial First Lady. Originally based out of New York, the ship fired the first shot of the Civil War as she tried to stop a ship inward bound to Charleston on the night before the bombardment of Fort Sumter on April 12, 18161." - Lincoln Paine, An Historical Encyclopedia of the Ships of the World