The clauses in the ratification documents only show that the right to reassume those powers was an accepted idea when the Constitutional union was created. They establish a principle applicable to all states, for if it was true for one, it would be true for another. It was an "equal" Constitutional union.
The problem with the "secession was legal" argument is that none of the southern states brought their case for secession before the US Supreme court....If amendment X of the US constitution enumerates a states right to secede, then Article III informs the state(s) where to go with their greivence.
They did not need to take anything to the Supreme Court because they were not disputing the constitutionality of any law or seeking legal arbitration to any dispute. They were excercising the previously stated right that a State could reassume the powers it had ceded to the Constitutional union. A right declared and included in the very documents that created the Constitutional union. Once they had reassumed those powers, they had also reassumed those powers previously granted to the Court.
This "the union is perpetual until we decide it isn't" mumbo-jumbo is logistical sleight of hand, that avoids the court and makes the confederates (old and new) look like the disreputable schemers is suspect they were.
LOL - Apparently you consider the Founding Fathers to be "schemers" spouting "mumbo jumbo" because they got rid of the "perpetual" Articles of Confederation ten years after creating them. You're starting to sound more and more like ol' waltrot every day. Soon you will no doubt do like him and call the Constitution a "pact with the devil". I would hope that is not the case and that you have merely been misled by him.