What you wrote about Lincoln wasn't even in history.Let me guess, what I wrote about Lincoln was not in the movie.
Shocking!!
...
You summed up your OP with the worthless tiddlebit of the Corwin Amendment and
"...but why then was the South so determined to leave the union, especially in light of the fact that they apparently did not need to end slavery as a price for staying in the union?"
altogether missing the fact most of the states had already seceded by the time it passed Congress.
Timelines. Facts. They matter.
Not when you're pushing an agenda. Libertarians are like Marxists that way.