I would argue that there's a difference between the reasons that led the nation to war in 1861 and the reasons that, over the course of the war and in the decades that followed, emerged as important.
I'd also argue that the binary "either/or" reasons you give aren't mutually exclusive.
Why were the slaveholders of 1776 any more entitled to nationhood than the slaveholders of 1861?
Ah, well there you run into political theory and philosophy. I tend toward a view that no one is "entitled" to nationhood, and that it's a construct established by facts on the ground, not by abstract notions of "natural law" or "entitlement." To claim otherwise is to open a can of worms in which any group, any area, anywhere can claim an "entitlement" to nationhood.
I don't fault the southern states for launching a rebellion and trying to establish their independence. What I object to is the notion that it wasn't a rebellion and was instead some irresistible legal process that no one had any right to oppose once they said the magic words.