I will agree that the Fort Sumter crisis was a matter largely of action and reaction. One side did something and the other side responded. Lincoln had no intents of resolving the situation peaceably short of the South Carolinians giving up. Which would mean their abandonment of secession and independence. Lincoln said so himself.

In my mind had Beauregard et. al. allowed Fort Sumter to have been reinforced unopposed that this would have signaled a crack in South Carolinas resolve and thereby reflect upon the resolve of all the other southern states. I believe that this was a part of the thinking at the time.

Now you did bring up a point as far as an option. What if the South Carolinians had armed a couple of boats/ships and placed them at the harbor entrance and Blockaded the entrance themselves? Had the South Carolinians declared that Charleston Harbor was closed to all unauthorized traffic, would that have forced the United States navy into a confrontation at the entrance?

I know that there were southern patrol boats at the entrance because the relief fleet was concerned about avoiding them and running past them. But, I am not sure that they were armed. And I don't think that South Carolina had announced the closure of the harbor. Did South Carolina leave the legal door open for Lincoln? The Harbor could have certainly been declared unsafe because of the situation prior to April 12th.

My thinking is if the relief fleet could not have entered the harbor without a fight would Lincoln have risk the expeditions. Certainly Anderson garrison was not able to hold out much long without that relief.

Minor points I know, but legalism often rest upon such minor points.