And so it may have been, but by the time the Morrill bill was turned into law, it was one of the largest tariffs in decades. It had moved well beyond a simple revenue tariff to a protection tariff - an economic distinction that was certainly known to them in the mid 19th century.
Had Southern leaders kept their heads about them, there was no reason to expect massive tariff increases.
There was the Morrill bill. It was well on the way to passage before any southern state seceded, and Lincoln gave every indication he would forward its passage as a protectionist measure well beyond revenue collection.
Indeed, according to one source, there was no effort to get Buchanan to commit himself to vetoing the tariff
The tariff passed the Senate on March 2, 1861. Secession was already a done deal in the majority of the souther states by that time. They saw their affiliation with the US as having been ended and had no reason to continue to lobby its president.
and Robert Toombs voted for the tariff, either because it wasn't regarded as harmful, or because it could be used as a pretext for secession and a provocation to Britain to condemn the unionists.
Toombs' left the Senate sometime in January. The vote on the tariff was on March 2nd.
Similarly, a President who announced in advance his intention to veto legislation were in injurious to the Republic can have a major effect on the kind of legislation that Congress produces. Had an effort been made in advance to lobby Buchanan, it would clearly have had some effect.
Beyond that, in those days, 2/3 of Senators had to vote to cut off debate and bring a measure to a vote. A Southern filibuster would have defeated the Morrill tariff, or any other legislation that was found to be injurious to the South. You chose not to respond to this point, but it is an important one. The tariff would not have become law had Southern states remained in the union. Secession was the cause of the tariff not vice versa.
Similarly, a 2/3 vote of the convention was required in those days to nominate a Democratic Presidential candidate. There was no reason to split the party as a compromise candidate could have been found, had Southern delegates not left.
Procedural safeguards were in effect to protect minority (i.e. Southern) interests. As of 1860, the system worked. It was the Deep South that chose not to work within the system.