I'm not sure I understand your argument as it applies to Texas. Texas voters elected representatives to a secession convention endorsed by the state legislature. The elected delegates to the secession convention then voted to secede, subject to confirmation by the voters of the state themselves. The voters of the state then overwhelmingly voted to secede.
The Texas procedure used the same steps that the original states did to ratify the US Constitution and, in fact, went even further by submitting the secession question directly to the voters themselves.
We the delegates of the people of Texas, in Convention assembled, have passed an ordinance dissolving all political connection with the government of the United States of America and the people thereof and confidently appeal to the intelligence and patriotism of the freeman of Texas to ratify the same at the ballot box, on the 23rd day of the present month.
But perhaps you are quibbling with the words used in the Texas secession documents. Perhaps the words didn't say "withdraw from the Constitution". If that is your complaint, then you are quibbling and splitting hairs. The Texas documents said that the 1845 act of the people of Texas by which Texas joined the Union was repealed and annulled and that Texas sovereignty was resumed.
Texas would seem to be following the guidance of James Madison, father of the US Constitution, who said:
It appears to your committee to be a plain principle, founded in common sense, illustrated by common practice, and essential to the nature of compacts, that, where resort can be had to no tribunal, superior to the authority of the parties, the parties themselves must be the rightful judges in the last resort, whether the bargain made has been pursued or violated. The Constitution of the United States was formed by the sanction of the states, given by each in its sovereign capacity. It adds to the stability and dignity, as well as to the authority of the Constitution, that it rests on this legitimate and solid foundation. The states, then, being the parties to the constitutional compact, and in their sovereign capacity, it follows of necessity, that there can be no tribunal above their authority, to decide in the last resort, whether the compact made by them be violated; and, consequently, that, as the. parties to it, they must themselves decide, in the last resort, such questions as may be of sufficient magnitude to require their interposition.
No. The state ratification declarations I was referring to were those made by the states ratifiying the US Constitution. Texas belonged to Mexico then, so it doesn't really apply.
The problem with the "secession was legal" arguement is that none of the southern states brought their case for secession before the US Supreme court.
I think they may have gotten a favorable ruling from the court, especially Virginia which had expressed reservations in their ratification declaration.
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.
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.