No. You can't have it both ways.
Following the principles you espouse, it was the job of the British Navy.
“You say tomato, I say tomäto” eh rusty? I could recount a lost causer axiom about “the hit dog howls” but I would be more polite.
They had no business “inspecting”. That was the job of the US Navy or the Coast Guard. So, OK - if it was acceptable for insurrectionists to be “inspecting” on the Mississippi, then it should also be entirely acceptable for an established and legitimate organization such as the US Navy to be “inspecting” outside of the Charleston Harbor.
Tomato and tomahto is right. You say insurections, I say states that had a right to secede. So, we get back to the argument about secession, which was not prohibited by the US Constitution but was covered by the Tenth Amendment as well as the ratification documents of several states.
Mississippi had seceded. So had South Carolina. They were no longer in the United States. But perhaps you don't believe in the Tenth Amendment or in what the ratifiers said about what the Constitution meant. Madison however did. He said the following about how the Constitution should be interpreted (Source: Madison's letter to M. L. Hurbert, May 1830):
But whatever respect may be thought due to the intention of the Convention, which prepared & proposed the Constitution, as presumptive evidence of the general understanding at the time of the language used, it must be kept in mind that the only authoritative intentions were those of the people of the States, as expressed thro' the Conventions which ratified the Constitution.
You apparently belong to the camp of those who want to coerce other states to the point of having a civil war. As Hamilton warned in the New York Ratification Convention:
It has been well observed, that to coerce the States is one of the maddest projects that was ever devised. A failure of compliance will never be confined to a single State. This being the case, can we suppose it wise to hazard a civil war? Suppose Massachusetts or any large State should refuse, and Congress should attempt to compel them, would not they have influence to procure assistance, especially from those States which are in the same situation as themselves? What picture does this present to our view? A complying State at war with a non-complying State; Congress marching the troops of one State into the bosom of another; this State collecting auxiliaries, and forming, perhaps, a majority against its federal head. Here is a nation at war with itself! Can any reasonable man be well disposed towards a Government which makes war and carnage the only means of supporting itself -- a Government that can exist only by the sword? Every such war must involve the innocent with the guilty. This single consideration should be sufficient to dispose every peaceable citizen against such a Government.
I suspect we know which side of Hamilton's statement you favor.
Lincoln's blockade of the Confederacy was an Act of War. That is why SCOTUS recognized Lincoln's blockade proclamation as The start of the war, rather than the attack on Fort Sumter. Clashes of arms such as what happened at Fort Sumter may or may not be considered the start of war.
In fact, consider the steps that Lincoln took to provoke war and his sending of a battle fleet to South Carolina with the stated intention of forcing their way into South Carolina's waters if opposed. The following post of mine and the links it contains might help you understand: [Link]
No. You can't have it both ways.
Following the principles you espouse, it was the job of the British Navy.