Secession and ratification are ultra vires the Supreme Court. The Supreme Court can't overturn, say, Mississippi's ratification of the 13th Amendment (which they gave belatedly, a few years ago). They simply have no say in such matters.
Likewise, their later importunity notwithstanding, secession conventions, their acts of secession, and the voters' ratifications of same are all untouchable by the Supreme Court since they are transacted at the level of the People as Sovereign, acting under their supreme power to make and unmake constitutions and unions, to make new governments and unmake them again as the People see fit.
The pay grades decline, as it were, from the People to the People's Constitution to the officers of the U.S. Government; the Constitution is their creature and thing; they own it absolutely and outright and can do any thing that they please with it.
The Constitution in turn creates, and is supreme over, the Union. The Constitution controls every aspect of the Union, and the Union in turn, through the Congress, controls the United States Government, which is likewise a creature of the Union and controlled by the Constitution.
The Supreme Court, as an organ of the United States Government and a creature of the Constitution, is subordinate to the Union, which in turn is the servant -- slave! -- of the People.
Therefore the Supreme Court has no power to tell the People, their master, that the People may or may not ratify, amend, or unmake the Constitution.
Or secede from the Union, if it comes to that.
“Secession and ratification are ultra vires the Supreme Court. The Supreme Court can’t overturn, say, Mississippi’s ratification of the 13th Amendment (which they gave belatedly, a few years ago). They simply have no say in such matters.”
I guess we will never know.