That's a new one on me. I've read the entire Constitution front to back and I haven't found where it says that the Preamble can't be used to form the basis for the court's decision. You've shown that you can make up new paramaters for guiding Supreme Court decisions, and you've shown where you do not disagree with Texas v. White, but you haven't shown where the decision is invalid or illegal and you havent shown where anything has come along to overrule or modify the decision. And unless you can then the simple fact of the matter is that unilateral secession as practiced by Texas in particular and the southern states in general was illegal.
There is nowhere in the preamble to the Constitution any reference to the perpetuity or indissolubility of the union.
"You've shown that you can make up new paramaters for guiding Supreme Court decisions,.."
I have done nothing of the sort. What I have done is to show that the decision in Texas v. White is based on doctrines not contained in nor implied by the Constitution. As a result the decision was contingent and could not have been anticipated prior to its issuance. Whatever Constitutional law may be regarding such a situation, and I no more claim expertize in Constitutional law than I would presume to "set up new parameters for guiding Supreme Court decisions" I have to opine (strictly personally that is) that to make a contingent decision retroactive is unjust for exactly the same reason that making an ex post facto law is unjust.