If you are correct that southern farmers were no more hurt than midwestern farmers, why did the South pay 87% of the total tariffs in 1860? The South out-produced the midwest and the north combined?
I don't know about Southerners' purchasing 87% of the goods imported into the USA. But there's no difference between the situation of a Minnesota farmer and a non-slaveowning farmer in Georgia. Both were subsistence farmers insulated from the market, mainly growing what they had to survive on. I saw a copy of the will of my great great grandfather in Georgia and there's no possessions there that would have been imported from any farther than the iron works in Richmond.
To admit that tariffs were a factor in secession is to admit that the slave owning minority who grew almost all the export cash crops had an influence beyond their numbers. The question then becomes whether the poor southern masses fought for the rich man's slavery or for the rich man's personal prosperity. In either case, the idea of the poor masses fighting for the rich class's narrow well-being is a grotesque violation of both common sense and American ideals. This contradiction is why the Confederacy folded up so quickly under duress where a legitimate revolution of the people would have persevered.