The Morill tariff should be viewed in the context of a 50 year tariff regime whose effect was disproportionate and inequitable from a Southern perspective. Viewed in this context, the Morill tariff was seen in the Deep South as the last link in a chain of depredations from the North. That would have made it more like the first, than the last link. And it's a backwards way of looking at history. Rather than the tariff making the war, it looks like the war made the tariff. Certainly the fifty years of Republican dominance and high tariffs were the product of the war, and of the conflict over slavery that preceded it. It was secession and war that turned a minor revision of the tariff into something much more drastic and long-lived.
Lincoln's threat to use force had a more subtle objective: by threatening to use force Lincoln repudiated 100 years of Constitutional scholarship about a sovereign State's right of secession. Lincoln's message to South Carolina and other secession-leaning states was clear: we do not recognize your right to secede and we will restrain you by force from doing so. All of the Founding Fathers would have found Lincoln's novel legal doctrine of the Union "creating" the sovereign states to be outrageous and tyrannical
Most Americans probably had not considered the question of secession prior to 1860 and among those who did there was much disagreement. If you listen to latter day Confederates they will tell you that unilateral secession was universally assumed to be valid, but that's far from the truth. Many disputed such an idea. And even those who did accept it didn't presume that whatever the seceders did would be right. The idea of the union creating states was hardly "outrageous and tyrannical." The federal government played an important role in acquiring and settling territory and would-be states had to petition the federal government for admission.
Lincoln was not the great emancipator, he was the great centralizer.
I would say that he was the great emancipator and the great preserver. Of course it was the 13th Amendment, passed after Lincoln's death and ratified in part as a tribute to him that finally freed the slaves. But if one must attach a name to emancipation, who would be more appropriate? True political centralization would come later with the progressives and the New Deal. Railroads and corporations also promoted a national market. A Confederate victory would have been a setback for centralization in the form that we've experienced it (though Davis had shown centralizing tendencies of his own), but Lincoln is more in continuity with Washington, Madison, Jackson and others, than a new departure. He shared with the Founders a knowledge of the dangers disunion could bring. Nationalism didn't begin with Lincoln, nor did the welfare state.
"Rather than the tariff making the war, it looks like the war made the tariff...."
If President Harrison had not died suddenly in office in 1841 it is likely that another secession crisis would have arisen at that time. The Whigs (forerunners to the Republicans) were dominant in Congress at that time and stood poised to implement all the key aspects of the American mercantile system (National Bank. high protective tariffs, infrastructure projects). The accession of Tyler to the Presidency prevented this scenario from being played out as he turned out to be a stealth Jeffersonian. It nevertheless remains a fact that Southern politicians had began to complain vociferously about tariffs as early as 1824. Secessionist sentiment had already begun to build with the election President Harrison in 1841, at time at which the slavery issue was of a decidedly secondary importance on the national agenda.
"Most Americans probably had not considered the question of secession prior to 1860 and among those who did there was much disagreement...."
There were three serious secssion attempts prior to the Civil War. These secession crises all ocurred during the Jefferson and Madison administrations (1801 - 1817); they all involved foreign policy issues (Louisiana Purchase, Trade Embargo of 1807, War of 1812). The New England Federalists first debated secession in 1803 under the leadership of Senator Pickering. The content of this debate never even considered the possibility that a sovereign state lacked the right of secession. The New England Federalists consideration of secsssion heated up again after Jefferson completed the Louisiana purchase. Aaron Burr was recruited to run for Governor of New York with the implicit promise that he would add New York to the group of secessionist states. Burr's subsequent loss of the Governors race put a damper on the secessionist movement of the New England Federalists for a time.
The New England Secessionist movement gained momentum again with with onset of the War of 1812. The secessionist states held a convention of scession in 1814 in Hartford, Connecticut. Ultimately the scessionists did not gather enough votes to carry the secssion motion but it was close run affair. It was unquestioned among these delegates that a sovereign state possessed the right of secession from the Union.
Southern politicians in 1860 used almost identical rhetoric in justifying their decision for secession as the New England Federalists had. Their understanding of the Union as being a voluntary association of sovereign states coindided exatly with the understanding of this issue conveyed by Jefferson Madison, and Hamilton.