Yes it is.
That is not in the North.
No it is not. And your point is?
No country would allow another to occupy it’s main harbor.
Charleston was hardly the main harbor. It wasn't even the second busiest.
If you can’t see the point about Fort Sumter controlling Charleston Harbor then you are a dishonest person.
You actually might be right about New Orleans having most of the cotton trade but I don’t know. I do know that Charleston was a very important Southern port. Maybe the most important one.