What you've posted doesn't especially need to be refuted. People can judge whether you know better than Aleck Stephens and Professor Nevins.
It's you who cannot refute the fact that there was no noise whatsoever on tariffs after the shooting started, and relatively little before.
The cause of the war was slavery.
Here is the Democratic party platform. Find the tariff language:
NATIONAL DEMOCRATIC (BRECKINRIDGE) PLATFORM, ADOPTED AT CHARLESTON AND BALTIMORE, 1860.
Resolved, That the Platform adopted by the Democratic party at Cincinnati be affirmed, with the following explanatory Resolutions:
1. That the Government of a Territory organized by an act of Congress, is provisional and temporary; and during its existence, all citizens of the United States have an equal right to settle with their property in the Territory, without their rights, either of person or property, being destroyed or impaired by Congressional or Territorial legislation.
2. That it is the duty of the Federal Government, in all its departments, to protect, when necessary, the rights of persons and property in the Territories, and wherever else its Constitutional authority extends.
3. That when the settlers in a Territory having an adequate population, form a State Constitution, in pursuance of law, the right of sovereignty commences, and, being consummated by admission into the Union, they stand on an equal footing with the people of other States; and the State thus organized ought to be admitted into the Federal Union, whether its Constitution prohibits or recognizes the institution of Slavery.
4. That the Democraty party are in favor of the acquisition of the island of Cuba, on such terms as shall be honorable to ourselves and just to Spain, at the earliest practicable moment.
5. That the enactments of State Legislatures to defeat the faithful execution of the Fugitive Slave Law are hostile in character, subversive of the Constitution, and revolutionary in their effect.
6. That the democracy of the United States recognize it as the imperative duty of this Government to protect the naturalized citizen in all his rights, whether at home or in foreign lands, to the same exent as its native-born citizens.
Whereas, one of the greatest necessities of the age in a Political, Commercial, Postal, and Military point of view, is a speedy communication between the Pacific and Atlantic coasts; therefore, be it Resolved, That the Democratic party do hereby pledge themselves to use every means in their power to secure the passage of some bill to the extent of the Constitutional authority of Congress for the Construction of a Pacific Railroad from the Mississippi River to the Pacific Ocean, at the earliest practicable moment.
Transcribed and reverse-order proofread by T. Lloyd Benson from the Tribune Almanac, 1861, pp. 31-32; (facsimile edition: The Tribune Almanac for the Years 1838 to 1868, inclusive, comprehending the Politician's Register and the Whig Almanac, (New York: Published by the New York Tribune, 1868).)
There isn't a word about tariffs, because tariffs were not an issue.
Walt