Better study up on the Founders. From the Ratification of the Constitution by the State of New York; July 26, 1788 [my bold below]:
WE the Delegates of the People of the State of New York, duly elected and Met in Convention, having maturely considered the Constitution for the United States of America, agreed to on the seventeenth day of September, in the year One thousand Seven hundred and Eighty seven, by the Convention then assembled at Philadelphia in the Common-wealth of Pennsylvania (a Copy whereof precedes these presents) and having also seriously and deliberately considered the present situation of the United States, Do declare and make known. ...
That the Powers of Government may be reassumed by the People, whensoever it shall become necessary to their Happiness; ...
... Under these impressions and declaring that the rights aforesaid cannot be abridged or violated, and that the Explanations aforesaid are consistent with the said Constitution ... We the said Delegates, in the Name and in the behalf of the People of the State of New York Do by these presents Assent to and Ratify the said Constitution.
Hamilton and future Chief Justice John Jay voted for the ratification document that contained those statements above.
The legality of unilateral secession is clear – the Constitution does not allow it.
Please show me where secession is outlawed in the Constitution.
Madison said the following on July 24, 1788 to the Virginia ratification convention [my bold and underline below]. Madison and future Chief Justice John Marshall [along with three other Federalists] wrote something similar into the Virginia ratification document and then voted along with a majority of the Virginia convention to pass that ratification document.
That resolution declares that the powers granted by the proposed Constitution are the gift of the people, and may be resumed by them when perverted to their oppression, and every power not granted thereby remains with the people, and at their will. It adds, likewise, that no right, of any denomination, can be cancelled, abridged, restrained, or modified, by the general government, or any of its officers, except in those instances in which power is given by the Constitution for these purposes. There cannot be a more positive and unequivocal declaration of the principle of the adoption — that every thing not granted is reserved. This is obviously and self-evidently the case, without the declaration.
Secession was not prohibited in the Constitution. The power to prohibit secession was not delegated to the federal government or to other states that might oppose the secession of a given state. Secession remained in the powers reserved to the states or the people.
You know as well as I do that statements such as you bolded are nice sentimentality but hold absolutely no force of law.