The first thing we need to remember is: when the Alien & Sedition acts were first proposed in 1798, there was a genuine threat & fear of war with France, the proposed acts were considered legitimate, and were not opposed by Vice President Jefferson.
Only later, when the threat of war disappeared, and the acts no longer seemed necessary, did Jefferson see political advantage in opposing them.
Second, scour all the Founders' quotes you wish, but you will not find any which support secession "at pleasure".
Instead, all imply the existence of one or both of two conditions:
Indeed, when President Jefferson suspected his own Vice President, Aaron Burr, of plotting to have Louisiana secede, Jefferson sent troops to have Burr arrested and tried for treason!
So we know for certain what Jefferson believed about secession "at pleasure".
But in November 1860, when the Slave Power began organizing for secession, there was no mutual consent, and no material breech of contract, only the 100% constitutional election of "Black Republicans" and "Ape" Lincoln.
Yes, your quote from De Toqueville applies here -- so there was no war started by the Union against Confederates, no battles, no invasions, not one Confederate soldier killed, until, until,..
Until the Confederacy provoked war, started war, formally declared war on the United States, and sent its forces to support secessionists in Union states.
Bottom line: secession did not cause Civil War, nor did forming the Confederacy.
Civil War began because the Confederacy started & declared it, period.
You seem to think that the South seceded just for their own pleasure. That would be ridiculous. They obviously considered the issues important enough to leave the Union over. IN the Declaration of Independence it says that that "governments re instituted among Men, deriving their just powers from the consent of the governed." The key word is "consent". The Founders knew that the only legitimate use of governmental power is through the free and unfettered consent of the people governed. Without this consent the government has no moral right to exist. The governed, according to the Founders, have a right to a government that disciplines itself to the will of the people. The people posses an inherent right to dispose of any government that does not rule with the consent of the governed. When Abe Lincoln was elected, it was by only 39% of the vote that he won, and it was Northern states only that voted for him because the republican party, unlike all other political parties at the time or before, was a purely sectional party and only represented Northern interests.
The States, being sovereign, did not delegate or give up the right of secession when the joined the Union. And after all, all rights not delegated are retained by the States (amendment 10). In fact, the states demonstrated the right of secession twice before they joined the Union under the current constitution. First, in their secession from England, and second in their secession from the Articles of the Confederation. Remember, not all the States adopted the new Constitution at once. In fact the delegates were not even supposed to be writing a new constitution, but rather amending the old one. Instead, they wrote a new one and some of the states seceded form the old one and joined the new. The states that didn't secede remained in the old union for a few years before joining the new union. St George Tucker wrote that "And since the seceding states, by establishing a new constitution and form of federal government among themselves, without the consent of the rest, have shown that they consider the right to do so whenever the occasion may, in their opinion require it, we may infer that the right has not been diminished by any new compact which they may since have entered into, since none could be more solemn or explicit than the first, nor more binding upon the contracting parties."
The State of Virginia, when it ratified the Constitution stated in their Act of Ratification that:
"We, the Delegates of the people of Virginia, duly elected....in the name and in behalf of the people of Virginia, declare and make known that the powers granted under the Constitution, being derived from the people of the United States may be resumed by them whensoever the same shall be perverted to their injury or oppression, and that every power not granted thereby remains with them and at their will: that therefore no right of any denomination, can be cancelled, abridged, restrained or modified."
The Federal government began taking more power for itself and abridging the rights of the states from very early on. Thus Madison and Jefferson wrote in the Kentucky and Virginia resolutions that "Whensoever the general government assumes undelegated powers, its acts are unauthoritative, void, and of no force. That to this compact each State acceded as a State, and is an integral party; that this government, created by this compact, was not made the exclusive or final judge to the extent of the powers delegated to itself; since that would have made its discretion, and not the Constitution, the measure of its powers; but that, as in all other cases of compact among parties having no common judge for itself...each party has equal right to judge for itself."
Milton supported the right of secession and self-government. In his writing The Tenure of Kings and Magistrates, he wrote that "the power of kinds and magistrates is nothing else but what is only derivative, transferred, and committed to them in trust, the right remaining in [the people] to reassume it to themselves, if by kings or magistrates it be abused."
The States, as sovereign entities, have every right to reassume powers delegated when they deem them abused. William Rawle, born in 1759 in Pennsylvania, wrote in a textbook published in 1825 that was used at West Point and many other colleges, that
"It depends on the state itself to retain or abolish the principle of representation, because it depends on itself whether it will continue a member of the Union. To deny this right would be inconsistent with the principle of which all our political systems are founded, which is, that the people have in all cases, a right to determine how they will be governed."
Regarding fort Sumter.....Yes Lincoln had made promises. For one, he promised the State of Virginia that Fort Sumter would be evacuated if Virginia did not secede. And so Virginia voted down secession (she voted in favor of secession later on when Lincoln wanted her to supply troops to fight her southern brethren). The South also sent delegations to Washington, D.C., and offered to pay for the Federal properties and enter into a peace treaty with the United States. Lincoln rejected negotiations. Finally, Lincoln told South Carolina that he was going to be sending down provisions for the Fort. It was only when the discovery was made that he was also sending arms and reinforcements that the decision to attack was made.