I think the point that people are trying to make is that it does not matter whether South Carolina legally seceded or not. If secession was illegal, then clearly firing on Fort Sumter was illegal.We can find plenty of examples of foreign toeholds in a country's territory. But the reality of the country was not in question, as it was in 1861. If Lincoln had said "boys, secede, it's fine with me, but I want to keep Ft. Sumter." most secessionists would have said "hell, yes!" to that deal.
But what Lincoln was saying is: "secession isn't going to happen. The federal government will not relinquish its authority over all the US. Starting with Ft. Sumter."
At the same time, the middle Southern states(especially VA) were wavering. Davis was leading an experiment. He thinks: "If we push at Charleston, Lincoln will push back. And the upper South will have a choice: shoot at Southerners or shoot at Yankees." He was correct that enough, if not all, would prefer to shoot at Yankees. So he gets most of what he wants, and a viable nation, if they could keep it.
He also overtly starts the shooting, and the indifference in the North vanishes. It would have anyway(preserving the Union was a goal not solely dependent on firing on Sumter), but Davis pulled a Pearl Harbor. An initial victory with long term consequences.
They argue, however, that secession was legal. If we assume that premise (which I believe to be clearly wrong), then they argue that South Carolina was within its legal rights to fire on Fort Sumter. The point we have been trying to make is that even if we accept their premise, firing on Fort Sumter was illegal in that it was an act of war by one sovereignty against another.
The point is to show that South Carolina is d***ed either way. Either their actions in firing at Sumter were an illegal act of rebellion against the legitimate government or were an act of war against the United States. Either way, Lincoln is not the "agressor" as they try to paint him. Either he is acting to supress a rebellion or he is responding to an attack against his country.
All of which puts South Carolina in the wrong no matter what you think of the validity or illegality of secession.