To be bluntly honest - I really would rather discuss this without refering to what the south did or didn’t do / should or shouldn’t have done / whether the south was justified , etc ... as I am NOT defending the south / slavery / or what have you ...
It is a purely theoretical debate whether states / the people can seceed from the union.
The state (as a political entity - e.g. any government) is NOT more powerful than the people because the people can ultimately disband the government at any time they so choose.
The government DOES act on behalf of the people - that is it’s entire reason for being - and should generally act in the people’s best interest! It is a general principle that a duly elected representative government is considered to reflect the will of the people.
You state that the founders were against a state leaving the union once they entered - and I would agree that they did not like the idea! However, I find no wording from them that actually indicates that it was forbidden to do so. Maybe I missed it ... OTOH it would seem to go against the very principles the nation was founded on and just about all writings of the founders regarding how the constitution was designed.
IMHO the founders dreaded the idea of secession as they KNEW it would ultimately lead to armed conflict as no nation ever voluntarily accepts the loss of a part and it would have meant the destruction of the very thing they held dear. But they were apparently UNWILLING to actually codifiy this as a prohibition as it would have scared the people / the states - no one at the time wanted a return to tyranny (excepting the odd torie or two ...). They knew that the MUST leave that option open in order to soothe the naysayers who contended that the Federalists wanted nothing less than a new monarchy.
This is WHY Madision is so obtuse in his writings about Factions - instead of stating the logical conclusion, that when the confederation of states FAILS to protect the rights of a minority, that the minority has retained the right to dissolve the bonds!
The people, acting on thier own, can never stand against a coordinated power. Instead, they exert thier rights through representatives who speak on thier behalf. Thus, when the people of a region decide that “enough is enough” - they express this through thier regional representatives. That is where a “State” derives it’s power to secede.
It is my contention that the constitution spells out plainly what sovereign powers the federal government was granted AND what sovereign powers the several States where prohibited in excersising. The Constitution does NOT specifically prohibit the power to secede to the several states (although several other sovereign powers are prohibited), nor does it grant the federal government the power to prohibit it! That alone - even without the 10th Amendment provisions, is sufficient to justify my contention that secession is “legal” (but I don’t want to be around when it happens!)