The States ratifying the Constitution didn't think it was. What was said after the fact does not matter!
Nothing in that says anything about secession or how is should, or could, be accomplished. Madison is on record as saying that a lawful secession requires the consent of all the states. The Southern states did not try that route when they chose to leave.
Can you provide anything other than provide letters stating such? No. Madison is on record in Federalist #43 stating this:
The first question is answered at once by recurring to the absolute necessity of the case; to the great principle of self-preservation; to the transcendent law of nature and of nature's God, which declares that the safety and happiness of society are the objects at which all political institutions aim, and to which all such institutions must be sacrificed. PERHAPS, also, an answer may be found without searching beyond the principles of the compact itself. It has been heretofore noted among the defects of the Confederation, that in many of the States it had received no higher sanction than a mere legislative ratification. The principle of reciprocality seems to require that its obligation on the other States should be reduced to the same standard. A compact between independent sovereigns, founded on ordinary acts of legislative authority, can pretend to no higher validity than a league or treaty between the parties. It is an established doctrine on the subject of treaties, that all the articles are mutually conditions of each other; that a breach of any one article is a breach of the whole treaty; and that a breach, committed by either of the parties, absolves the others, and authorizes them, if they please, to pronounce the compact violated and void.
Madison later defended his statement with his Virginia report:
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 upon 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.
You may say, "Federalist #43 is in regards to the Articles of Confederation" -- Remember, the Articles have language of eternity, the Constitution does not.
Please see #306 above.
Respectfully that's not what you said. And in order to support your claim you quoted gentlemen speaking long after the Constitution had been ratified. Are you now saying that their opinions did not matter?
Can you provide anything other than provide letters stating such? No. Madison is on record in Federalist #43...
What was Federalist #43 but a letter expressing an opinion? Besides, nowhere in Federalist #43 does Madison mention the mechanics of ending the compact. He did in later letters.
Madison later defended his statement with his Virginia report...
Madison also said later that his writings in the Virginia report in no way indicated support for the idea that a state could secede at will from its union with the other states.