Should have, could have, would have. There was no right to unilateral secession and to even maintain that position in the face of direct statements of the framers is to see those framers as hopelessly naive pontificators with no sense of political reality. The Constitution was written and ratified with all the means and authority necessary for the congress and executive to compel allegiance through force.
The south could have easily withdrawn from the union if they had followed constitutional procedures. But that would have entailed treaties establishing recognized national borders and no small expense upon the departing states to make their sister states whole for the common investments. The south's intention was not a peaceful separation but the establishment of a government with sufficient military strength to repudiate debts and to claim western lands for the expansion of slavery that they could not get through the electoral process. They sought to do with force and threats of force what they could not achieve at the polling place. That, by any definition, is a common rebellion.
The secessionists thought the yankees couldn't/wouldn't fight. But they ordered up an army of 100,000 just to make sure they could duck out on their obligations.
Walt
Excellent comment. Well worth repeating.