To reply to your post, "immediate cause" I understand just fine, but it isn't the same thing as "main cause", "root cause", "underlying cause", or anything like that. Yes, the slavery issue had been pushed by Republican and Abolitionist propagandists until everyone was acting like a Jerry Springer audience (Jerry Springer is evil, bring me the head of Jerry Springer), and they were all ready to start killing each other.
But here is the thing. Control of agenda, control of one's own political destiny, is far more potent a first cause and source of motive than any individual issue, because control goes to ALL issues and so is as powerfully motivating as the sum of all issues in play.
Control, rather than any one issue, even a big one like the basis of the Southern economy, is the really big motive in secession, and if you would take the time to review Stephens's other speeches such as the one I linked to in my post to x above, you would see that control links like a fetter all the issues he mentions either in the Cornerstone Speech or his speech to the Georgia legislature a few months earlier .
You can't raise a control issue like the one in play with Lincoln's election, without simultaneously setting into motion all the other causes and issues that pressed upon the actors of that day.
That there were other complaints that the South had does not change the fact that it was slavery that was driving the move for secession.
This was the motivation for the deep south states.