And it would have been a simple matter to physically block the narrrow passage into Charleston harbor to prevent any resupply effort. A string of rafts connnected by a heavy chain sufficed to keep the British from sailing up the Hudson in 1778. A similar arrangement in Charleston would have passively changed the burden of taking the first hostile action onto the Union. Anderson would then have had no choice but to leave the fort on the 15th, as he'd earlier told Beauregard.
A claim could also be made that the South (see, I capitalized it) backed Lincoln into a corner just as effectively, and in the short term got more out of it since Lincoln's inevitable reaction to Sumter--calling for volunteers--had the effect of pushing the upper South to secede, something that most of Lincoln's actions up to that point had been designed to avoid.
The power of ships was much greater in 1861 than it was in the Revolutionary War. Here is General Beauregard's Oct 8, 1862, opinion:
I have applied for fifty-one additional pieces of ordnance of the heaviest calibers to arm the inner line of forts and batteries. I consider them indispensable, for my reliance in the boom and other obstructions now being laid across the channel between Fort Sumter and the new batteries on Sullivan's Island is but very limited, except for their moral effect.
Jefferson Davis had the following opinion:
The main advantage of the obstructions across the harbor of Charleston is that they may prevent the enemy's gunboats from running rapidly by during the darkness or at other times. If they are sufficiently strong to detain the vessels for a considerable time under the fire of our forts and batteries, and the guns are well served, we should be able to sink or drive away any boats that might attempt to pass.
The Feds had sunk old whaling vessels containing stone in the main cannel at Charleston in December 1861 and 14 vessels in one of the other channels leading into Charleston harbor in 1862 (the "Stone Fleet"). These proved ineffective. "... in a short time not a vestige of any of the stone fleet could be found; it sank slowly in the mud, where it still lies." [Source: The Siege of Charleston 1861-1865 by E. Milby Burton, page 89, paperback version] Also, currents quickly reformed channels to the harbor.
Blocking the main channel with sunken ships full of stone had been tried by the Governor of South Carolina after the Star of the West incident and before the firing on Fort Sumter. "On the 11th [of January, 1861] he [Governor Pickens] had four ship hulks loaded with stone towed out and sunk to block the main channel at the harbor entrance." [Source: "Days of Defiance," by Maury Klein, page 261 paperback version]
In April 1861 the Confederates launched quite an effort to develop mines and floating contact torpedoes to block Federal ships. A torpedo station was later located at Charleston.
After Sumter, the Charleston harbor obstructions used by the Confederate included contact mines, electrically controlled mines that could be detonated from shore, booms extending across the channel, ropes in the water to foul the propellers of Federal vessels, piling placed in the shallower parts of the harbor. [Source, Burton's book cited above, page 269] These obstructions keep the Federal fleet at bay for over three years, so it sounds like Davis was right.
All the while blockade runners were able to make it into and out of Charleston despite Federal picket vessels and obstructions.
Welles, for one.
State governments/governors/legislatures of Florida and South Carolina (sending spies into Pickens and Sumter).
His deception was so spread that he redirected Powhatan in the midst of her deployment to Sumter (or was it purposeful sabotage of his own mission, which initself constitutes an attempt at deceit???)
I have to dispute your idea. Do you have any idea how WIDE the entrance to Charleston Harbor is? NO WAY would they have been able to block entrance to the harbor. I know from personal experience on this one. I was in the Navy, and my sub passed that way everytime we went out to sea.