IMO, the decision to use force was reached when the South found out from contacts in the North that Lincoln was preparing to send a battlefleet down to SC territorial waters despite earlier promises to evacuate the fort from Lamon and intimations of that from Seward and leaks to Northern newspapers. Up until they learned Lincoln was preparing a battlefleet to sail, they allowed Anderson to buy food in the Charleston market.
As Anderson said when informed that the fleet was coming, "I trust that this matter will be at once put in a correct light, as a movement made now, when the South has been erroneously informed that none such will be attempted, would produce most disastrous results throughout our country.
I'm not sure what Davis and others thought about the threat the fort posed to the Confederacy's existence. My own guess is that they might have considered it a threat since the Confederacy's economic existence and balance of payments depended on export of cotton and tariff revenue from imports and the fort could be used to enforce Lincoln's announced intention to collect tariff revenue from Southern ports. Here Lincoln was sending a battlefleet down either provision or provision and reinforce (who could believe his promises by this point) a fort that could enforce that Lincoln's tariff collection scheme by threatening ships that entered the harbor without paying tribute to Uncle Abe. Winfield Scott's communications said the purpose of the expedition was to reinforce Sumter.
What was the old American saying, "Millions for defense, but not one cent for tribute?" Lincoln was fast becoming a Barbary pirate.
I've made the argument before that Lincoln instigated the war because he needed revenue to run the country and he couldn't do that without Southern exports that provided the great bulk (about 70-75%) of the United States balance of payments. So, in a key sense both sides fought the war over economic matters. Exports and imports were not the only reasons, of course. Slavery was the main reason the South seceded -- it was basic to their economy. Oops, that's another economic cause of the war.
How can that be? A supply effort had been attempted in January, and that was driven off in what was truly the first act of aggression on either side. Yet that hadn't been preceded or followed by a whole scale attack on the fort. And had Lincoln landed food at Sumter as he stated was his intention, what would have changed? Would the threat to the confederacy grown? No. If Sumter wasn't enough of a threat to bombard into surrender in January then how could that change in April? And had Lincoln lied and landed troops as well as supplies, would the confederacy been in danger? No. A few hundred men could not have taken the city, and while the forces in Sumter might, might, have blocked Charleston harbor, but that was far from the busiest port in the confederacy. New Orleans was five to eight times busier, and even Mobile exported more. Eighty percent of confederate exports and almost ninety five percent of their imports entered at ports other than Charleston. Exports could easily have been sent out through Savannah and the amount of imports was so small that even if those could not have been directed to Savannah or Jacksonville as well, the amount was a mere 1/20th of the total. So there was no economic threat had Lincoln kept the fort; the confederacy would have gone on without any problems. And as for Lincoln's pledge to collect the tariffs, how do you imagine he could have done that with Sumter alone? Ships didn't pay tariffs there, they paid them where they landed the goods. Lincoln could have blustered all he wanted, no tariffs would have been collected unless South Carolina allowed it. So your claim that an act of aggression on the part of Lincoln, real or perceived, caused the South to act in return just isn't supported. There was no threat that hadn't been in place for months. What really changed?
Nothing Lincoln did had to force the issue. The status quo worked more in the South's favor than in Lincoln's. The South acted because their attempt to starve the fort into surrender had failed. They attacked because every other attempt to force Lincoln to turn over the federal property had failed. They attacked because impatient and because the status quo no longer suited their purposes, and that Sumter was worth a war. It was that single act that doomed the confederacy. An independent South died on April 13, 1861. It just took 4 years for the body to hit the floor.