Rusty, you well know the key word here is "necessary", and no Founder, none, believed secession "at pleasure" was constitutional or lawful.
Indeed, secession "at pleasure" is the very definition of the word "rebellion", and every major Founder, without exception, took decisive actions against rebellion, insurrection, "domestic violence" or treason:
Clearly, our Founders fully understood the difference between lawful secession and rebellion, and were totally intolerant of the latter.
So, did Founders believe in lawful secession?
Of course they did, just as their new "more perfect" Constitution of 1787 "seceded" from the old Articles of Confederation "perpetual union".
So, what was lawful secession?
Well, Madison's considered opinion explained it best.
Madison said secession, or disunion, were valid under two circumstances:
Did either of those conditions apply in 1861?
No, not even close.
So those declarations of secession were "at pleasure".
Could either apply today?
Sure -- Congress could lawfully approve any state's request for secession, or a convention of states could carve out whole regions into separate countries.
What about unapproved, unilateral declarations of secession?
No, those remain today, as they were to our Founders, acts of rebellion, insurrection and/or treason.
2. President Adams passed (and Jefferson initially supported) Alien & Sedition acts in preparation for war against France, now known as the Quazi-War of 1798.
3. President Jefferson arrested and tried for treason his own former Vice President Burr, on suspicion Burr was going to lead Louisiana to declare secession (1807).
4. President Madison moved US Army troops off the war-time frontier with Canada to Albany to be in position to put down rebellion in case the Hartford Convention declared secession in 1814.
As a matter of historical interest, we should note that these examples don't support your case.
The Whiskey Rebellion was not a "secession," per se. It was simply a...rebellion.
The Alien and Sedition Acts had nothing to do with secession. They are also widely admitted to have been unconstitutional, which is why Congress repealed them shortly thereafter.
Burr's attempt to create his own western kingdom was not a secession because it did not involve a state or states trying to leave the union. Instead, Lousiana was a territory at this time, and the Constitution had implied, and the Northwest Ordinances clarified, that territories of the United States were under federal control. Burr's effort was not "secession" in the sense this discussion means it, but would have been more akin to him trying to steal federal territory outright.
The Hartford Convention example is perhaps the closest to an issue of secession with these, but still fails as an example because your use of it essentially begs the question.
Further, we ought to note that there were a lot of other factors, not least of which were sectional rivalries, involved in the preparations to suppress the Hartford Convention. What this means is that the move to do so dids not stem from some pure-as-the-driven-snow conviction on the part of "the Founders" that "secession was wrong," but instead involved a lot of rather petty partisan and sectional concerns that would have been in play regardless of the legal status or lack thereof of secession.
1.Mutual consent, meaning logically, approval by Congress or by Convention of the States.
2.Or "...by usurpations or abuses of power justly having that effect," meaning a serious breach of compact rendering its binding obligations null & void.
Did either of those conditions apply in 1861?
No, not even close.
So those declarations of secession were "at pleasure".
I know you don't think that nullifications of the Constitution by various Northern states blocking the return of fugitive slaves were not serious enough to justify secession. Perhaps you are forgetting that the return of fugitive slaves was voted for unanimously by the Constitutional Convention. That and other similar compromises between North and South helped make the Union under the Constitution possible. There was a lot of give and take between the two regions to get to the final Constitution.
According to the description of some of the horse trading at the Constitutional Convention by the authors (Edward J. Larson and Michael P. Winship) of "The Constituional Convention, A Narrative History from the Notes of James Madison":
On August 24, the eleven-member committee reported its recommendations to the Convention concerning the major outstanding issues relating to congressional power over trade. With respect to the slave trade, it recommended a compromise forbidding Congress to ban the importation of slaves until 1800. The following day Charles Cotesworth Pinckney of South Carolina moved that the date be changed to 1808. The Convention approved this motion, with New Jersey, Pennsylvania, Delaware, and Virginia voting no. With respect to statutes regulating international shipping, or "navigation acts," the committee recommended striking the southern-inspired restriction requiring two-thirds majority in each house for their passage, and the Convention passed it on AUGUST 29.
On August 28, as a concession to the South, the Convention unanimously approved a requirement that fugitive slaves in any state "be delivered up to the person justly claiming their service or labor."
As I've posted before, and I'm sure you know, Daniel Webster, who served twice as Secretary of State, and also as a Senator from Massachusetts, and a Representative from Massachusetts and New Hampshire, warned the North as follows in 1851:
If the South were to violate any part of the Constitution intentionally and systematically, and persist in so doing, year after year, and no remedy could be had, would the North be any longer bound by the rest of it? And if the North were deliberately, habitually, and of fixed purpose to disregard one part of it, would the South be bound any longer to observe its other obligations? I have not hesitated to say, and I repeat, that if the Northern States refuse, willfully and deliberately, to carry into effect that part of the Constitution which respects the restoration of fugitive slaves, and Congress provide no remedy, the South would no longer be bound to observe the compact. A bargain cannot be broken on one side and still bind the other side.
And yet, some Northern states continued after this warning to flout the Constitution. To save bandwidth, I'll link to one of my earlier long posts citing reports of what some Northern states were doing: Link