Wouldn't Northern consumers of those goods be paying the tariff as well?
Yes. I seem to remember that one of your former personas used to ask that question. Those who worked for Northern manufactures had jobs, some of which had been made possible by the existing protective US tariff. Northern manufacturers anticipated greater profits and probable future expansion of their businesses under the higher Morrill Tariff.
To remain competitive with lower priced European goods in the South, the prices of Northern goods would have to be lowered significantly below those supported by the Morrill Tariff. Northern goods sold in the South would have to pay the same tariff to the South as that placed on European goods by the Confederate tariff. This would put much price and profit pressure on Northern manufacturers. This meant less profit and likely fewer jobs for Northern workers. All of that would reduce the $200 million dollars of Northern goods sold in the South. Ripple effects would follow throughout the Northern economy.
The South initially set their tariff the same as the then current US tariff, then about two weeks later the North passed the much higher Morrill Tariff for the US. Talk about shooting themselves in the foot! That set up the two-tariff problem that prompted Northern newspapers to basically cry 'blockade the Southern ports', 'our pockets have been touched'. Northern import businesses started closing. The picture looked bleak for Northern tariff income.
Whatever percentage of the tariff had been paid by the South in the past, Lincoln certainly realized the problem the Northern economy and the government finances would be faced with because of the loss of the South and the two-tariff problem that the Republicans had created. As Lincoln said, "And what is to become of the revenue? I shall have no government -- no resources." [Link 1 and Link 2].
Lincoln was a smart guy. He thought outside the box and provoked a war so that he could invade the South and blockade their ports solving the two-tariff problem. The tariff was not the main reason the South seceded, but the tariff income problem was the reason why Lincoln started the war.
As Lincoln basically said in his inaugural speech, you can keep your slaves, but I want your tariff income.