[BroJoeK]

rustbucket: "...apparently their 1860-61 secessions did not meet your personal supra-constitutional requirements for secession where you get to define what is necessary."

Sorry, but it's not about me, or even about you, it is about what our Founders intended for their Constitution.
If you claim to be conservative, then it's about their definitions of "necessity" and "mutual consent", not yours, or even Jefferson Davis'.

As you well know, the best explanation I can find is Madison's letter, here.
Parts of that letter have been quoted on these threads many, many times.
In it Madison makes the case against secession at pleasure:

"Applying a like view of the subject to the case of the U. S. it results, that the compact being among individuals as imbodied into States, no State can at pleasure release itself therefrom, and set up for itself.
The compact can only be dissolved by the consent of the other parties, or by usurpations or abuses of power justly having that effect.
It will hardly be contended that there is anything in the terms or nature of the compact, authorizing a party to dissolve it at pleasure."

Madison also discusses the dire consequences of such disunion, then ends with another analogy:

"Even in the case of a mere League between nations absolutely independent of each other, neither party has a right to dissolve it at pleasure; each having an equal right to expound its obligations, and neither, consequently a greater right to pronounce the compact void than the other has to insist on the mutual execution of it. [See, in Mr. Jefferson's volumes, his letters to J. M. Mr. Monroe & Col. Carrington]"

As to defining the difference between "necessity" and "at pleasure", clearly the 1776 Declaration defines "necessity" where mutual consent is not required, and the 1788 Constitution ratifications define "at pleasure" where mutual consent is required.

So, when did political conditions more resemble those of 1776 "necessity" (requiring no mutual consent) and when of 1788 "at pleasure" -- which did require mutual consent?

1. Clearly, in 1806 when former VP Aaron Burr attempted to secede with Louisiana, that was disunion at pleasure, and President Jefferson had Burr arrested & tried for treason.

2. In 1814 President Madison did not consider New England conspirators at the Hartford Convention arguing for secession due to oppressive Federal policies (the embargo on trade with Britain) -- Madison did not consider such secession "necessary".
   Instead, Madison (who knew something about Founders' Original Intent) responded by moving US Army troops off the border with Canada to Albany in case New Englander threats should result in rebellion.
   Those threats of secession at pleasure ended with Col. Jackson's victory in New Orleans, and resulted in the dissolution of the old Federalist party.

3. By 1820, during President Monroe's Missouri Compromise, Southern threats of secession over slavery were always linked with predictions of civil war:
   "You have kindled a fire which all the waters of the ocean cannot put out, which seas of blood can only extinguish."
   — Representative Thomas W. Cobb of Georgia

   "If a dissolution of the Union must take place, let it be so!
   If civil war, which gentlemen so much threaten, must come, I can only say, let it come!"
   — Representative James Tallmadge Jr. of New York:"

   Nobody in 1820 said there was an unlimited "right to secession" over slavery without civil war.

4. In 1830, when South Carolina threatened secession over President Jackson's "Tariff of Abominations", Jackson ordered US warships and army units to Charleston Harbor, famously saying:
   "...please give my compliments to my friends in your State and say to them, that if a single drop of blood shall be shed there in opposition to the laws of the United States, I will hang the first man I can lay my hand on engaged in such treasonable conduct, upon the first tree I can reach.[65]"

   "John Calhoun, if you secede from my nation, I will secede your head from the rest of your body."

   "On October 19, 1832 Jackson wrote to his Secretary of War:
   "The attempt will be made to surprise the Forts and garrisons by the militia, and must be guarded against with vestal vigilance and any attempt by force repelled with prompt and exemplary punishment."

   President Jackson's 1832 proclamation on Nullification.

Clearly, none of the presidents believed in either an unlimited "right of secession at pleasure", or in the "necessity" of secession at their times.

Nor did either of Presidents Buchanan or Lincoln in 1861.

rustbucket: "Why did some Republicans, at least twice - in 1860 and 1861 - propose Amendments to the Constitution that would require acceptance by the central government or other states before a state could secede.
They knew that without such amendments that states could secede without permission.
The seceding states did not need the permission of the states that might be oppressing them by sectional aggrandizement or by nullifications of the Fugitive Slave Act."

The truth is that before the November 1860 election, there were no negative conditions that any Southern state considered "secession-worthy".
It was only with the constitutional election of Lincoln's "Black Republicans" that some Southerners suddenly cried, "OMG, we just can't stand this any more!"
And yet, literally nothing had changed, in November 1860 Southern Democrats still ruled in Washington, DC, as they had for nearly all of the previous 60 years.

That is the very definition of secession "at pleasure".

As for proposed constitutional amendments and "compromises" intended to preserve the Union: there were many such in 1860 & 1861.
None were considered necessary or viable enough to become law.

[rustbucket]

Sorry, but it's not about me, or even about you, it is about what our Founders intended for their Constitution.

I am citing what the ratifiers of the Constitution said was necessary to resume/reassume their own powers of government under the Constitution. You, on the other hand, are applying what was said in 1776 to the Constitution, which did not exist at that point.

1776 was more than 20 years before there was a Constitution, and in the interim the country had seen what worked poorly in the government under the Articles of Confederation. The Articles began to be drafted shortly after the Declaration of Independence. The founders of the 1776 period did a poor job of setting up a central government under the Articles. So, 20 years later a convention in Philadelphia drafted the Constitution to fix the problems the Articles had caused. States then individually and separately ratified the Constitution. In ratifying it, they defined what was necessary to reassume/resume their own governance under the Constitution, which was not what others said or might have intended in 1776.

[You quoting Madison in 1830 in a letter to Trist]: "The compact can only be dissolved by the consent of the other parties, or by usurpations or abuses of power justly having that effect.
It will hardly be contended that there is anything in the terms or nature of the compact, authorizing a party to dissolve it at pleasure."

Who is it that gets to decide if there have been usurpations or abuses of power, the states doing the abusing or the states that have been abused? Your answer would seem to be, 'the states doing the abusing.'

Here, from another 1830 letter by Madison [Madison to Hurlbut], is another Madison statement:

"But whatever respect may be thought due to the intention of the Convention, which prepared & proposed the Constitution, as presumptive evidence of the general understanding at the time of the language used, it must be kept in mind that the only authoritative intentions were those of the people of the States, as expressed thro' the Conventions which ratified the Constitution.

So, here is what Madison said in the 1788 Virginia ratifying convention:

"An observation fell from a gentleman, on the same side with myself, which deserves to be attended to. If we be dissatisfied with the national government, if we should choose to renounce it, this is an additional safeguard to our defence."

What about Jay and Hamilton (Madison's coauthors of the Federalist Papers) and the majority of the other New York ratifiers who voted in their ratification convention for the statement:

"That the Powers of Government may be resumed by the People, whensoever it shall become necessary to their Happiness"

And the New Yorkers followed up such statements by declaring in their ratification document:

Under these impressions and declaring that the rights aforesaid [rustbucket: like the "necessary for their happiness" statement] 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.

Madison had earlier made his own statement about happiness in his Federalist Paper #45:

"Were the plan of the [Philadelphia] convention adverse to the public happiness, my voice would be, Reject the plan. Were the Union itself inconsistent with the public happiness, it would be, Abolish the Union.