It was a legal statute. Statutes can be repealed by their originator - even those that claim to be unrepealable. South Carolina repealed its statutes with the United States by act of secession. That would seem to include the Fort Sumter statutes.
Yes it was. You have demonstrated nothing except that both letters gave, as the primary purpose of the expedition, the peaceful landing of supplies only and that force was to be used on if such a landing was opposed.
Wrong. I have demonstrated that one used active and conditionally explicit language. The other used passive and intentionally vague language. One said "if they don't let you in you are to open fire right then and there." The other said effectively "Let us in and we aren't going to open fire anytime soon." If you cannot see the difference between those statements then you are dishonest, willfully obtuse, or just plain stupid.
Nonsense. The agreement said nothing about title being in affect until the legislature decided otherwise. The title was transferred permanently to the U.S. without reservation, without condition. Any other arrangement would have violated the U.S. Constitution, not that South Carolina had any problems with that.
I have demonstrated that one used active and conditionally explicit language.
You have demonstrated nothing except that both, I repeat, both letters clearly and unequivocally state that the primary purpose of the expedition was the peaceful resupply of Sumter, and that force would be used only if the resupply was opposed. You can hold it upside down, play it backwards, look for hidden meanings all you want but you cannot change plain English.