"Whatever role tariffs had, and I won't deny that they were an additional cause of friction, was a mere sideshow to the cornerstone issue."
Bingo, spot on! Every leading 'normal' American historian have honestly attributed the origins of the Civil War has having its root triggering cause deeply based in the Confederates die-hard insistence on expanding their Cotton Empire, which was based on the demonic, barbaric, Nazi-like system of forced labour of over three million souls who languished in the sweltering heat & humidity - as slaves.
Notice this Howell Cobb referred to the Confederate insurrection as the revolution and he is not the only spokesman for the Rebellion Inc who utilized the term the revolution. What you stated he is totally correct as well. "If the fight was about tariffs, why would the white soldiers lose faith in the rebellion? It seems that the white soldiers of the Union didn't have such a problem with black soldiers fighting for the United States."
Notice how all the neo-confeds have a self-convicting habit of going out of their minds, viciously attack anyone & everyone who worked for abolition of slavery (abolitionists), which begs the question, if one is/was so vehemently against the abolition of slavery in 'Confederate' states, in the identical way this very same backward element is/was against abolishing Southern state mandated forced racial segregation, (Jim Crow "laws") the only logical conclusion is the "tariff" counter attack is nothing more then another smoke screen attempting to remove the main focus off their phobias & hatred.