The Morrill Tariff was the new tariff that was coming in. Lincoln favored it, and didn't want to see it repealed. Everybody knew tariff rates would go up, and nobody much wanted to stop that. The question was how much. There was plenty of room for compromise about that.
We don't know what would have happened if the Confederates hadn't fired on Fort Sumter. Maybe they would have attacked elsewhere. Maybe the Federals would have attacked them. Maybe there would have been a compromise solution.
My own theory is that Lincoln was biding his time, pretending that the country was still intact, and hoping that reconciliation would be possible. If all federal forts and property in the rebel states had been confiscated, it would be hard to maintain the idea that the union was intact, so Lincoln had to hold on to those forts.
That's only my theory. But I don't think it was secession as such that Lincoln or Northerners objected to, as the violent assault on the fort and the flag. People took such things more seriously back then.
That Southerners saw slavery as being threatened by the Republicans is a given. But that in itself was not enough to sever the Union. The Morrill Tariff could, as it would protect northern industry and 3/4 of the revenue collected would come from the South.
Tariffs were assessed on imports and whoever bought imported goods paid a price that reflected the tariff. Militant Southerners thought that because their region exported so much and brought in currency through exports that they were in effect paying the tariff.
But Southerners bought goods and services from Northerners. They spent some of their profits on that. That left them with less money to pay for imports and Northerners with more money to buy imports.
So it's by no means a given that Southerners would have bought the lion's share of foreign goods or paid the biggest share of the tariff. Nor was it impossible that Southerners could have used the tariff to build their own industry. Many Southerners did.
The causes of the Civil War are complicated and to reduce them to the simplicity of a single cause (slavery) that was not a major issue in 1861 is foolish beyond belief.
Depends on what you mean by "cause." Of course there are always multiple causes to any conflict. You can go on and on asking "and what caused that?" to the end of time. But if you want to know what caused the divide between two parts of the country and what made such divisions so impassioned to the point where war was possible, slavery is a very good answer. Certainly better than something like tariffs.
Here's what we know for certain:
So Lincoln was committed to peace, so long as the Confederacy did not start war.
But Davis was committed to start a war whenever he believed his "integrity and jurisdiction" were being assailed.
That's why there's no doubt, none, that Jefferson Davis is responsible for starting Civil War at Fort Sumter.