It is quite a stretch to claim that that case was the legality of secession on trial. It was about redeeming war bonds, and prosecution for such. No stretch at all. The defense's arguement was that since Texas had seceded and hadn't been readmitted then she wasn't a state and had no right to take her case to the Supreme Court. The legitimacy of the Southern acts of secession were very much an issue for the court to decide, and decide it they did.
A real case about the legal merits of secession was not held. The principles in the Civil War were never put on trail, in part, because FedGov did not want to have to argue their case in court. I disagree that you have proven your point.
On the contrary, let me quote from the Texas v. White decision: "When, therefore, Texas became one of the United States, she entered into an indissoluble relation. All the obligations of perpetual union, and all the guaranties of republican government in the Union, attached at once to the State. The act which consummated her admission into the Union was something more than a compact; it was the incorporation of a new member into the political body. And it was final. The union between Texas and the other States was as complete, as perpetual, and as indissoluble as the union between the original States. There was no place for reconsideration or revocation, except through revolution or through consent of the States...Considered therefore as transactions under the Constitution, the ordinance of secession, adopted by the convention and ratified by a majority of the citizens of Texas, and all the acts of her legislature intended to give effect to that ordinance, were absolutely null. They were utterly without operation in law. The obligations of the State, as a member of the Union, and of every citizen of the State, as a citizen of the United States, remained perfect and unimpaired. It certainly follows that the State did not cease to be a State, nor her citizens to be citizens of the Union."
Just because you don't agree with the decision doesn't mean it wasn't made.
I'll try once more. There is a difference in sighting something as part of an argument, even forcefully, and having a trial on that issue.
A trial on "is secession constitutional" would mean that one sides lawyers would be filing briefs sighting the relevant passages of the Constiution, case law, English Common Law, legal definitions, etc. to make the legal case for seccession.
That trial never took place. Trials where those issues would have been forcefully put forward, that begged out for trial, such as Jefferson Davis, never took place. He was indicted for treason, but the charges were dropped.
There are entire books about the pros and cons of the legality of secession. I have read them, and it changed my thinking on the subject. Perhaps you should do the same if you are interested in this topic. (As we all know, too, the Supreme Court has been known to be wrong or change their mind occassionally, too.)
The book: Constitutional History of Secession by John Remington Graham looks at the larger picture outside the USA.