You're on to something in role the Westward expansion played in this period. The term "Southern Rights" was explicitly about rights of slaveholders in the Western territories. Also, as Lincoln reminded Illinois voters in 1858, white settlers didn't want to contend with black people in the western lands, slave or free.
As for the tariff, Pam, how many imported products would you expect to find in Montgomery, Jackson and Chattanooga stores? How many Southern farmers came to town expecting to buy lace imported from Holland, or chocolates made in Switzerland? High tariffs (above 20%) aggrivated wealthy planters and politicians who wanted to buy an English-made shotgun or a piano for their daughters. Most subsistence farmers and even those who brought a little cotton to market could have cared less.
Also, knowing what you know about coastal trade --
Do you suppose a sea captain making port in Pensacola could find a way to avoid paying those pesky duties on cargoes from Havanna, Nassau or Barbados?