Except James Buchanan did not take a “hardline” against secession. And the words of his State of the Union speech documents he did not take a hardline.
If he had taken a hard line he might have sent his Navy on the prod to provoke a Fort Sumter incident. Or at least he could have contended that dissolution of the union could only be valid through unanimous agreement of all the states.
Buchanan wanted to preserve the Union, but so did a lot of people including a lot of southern leaders.
Many historians rank Buchanan as one of the worst U.S. presidents, if not the worst, because they view him as being
softline, not hardline. See link.
The argument in your post 289, well intentioned, falls apart. But not until the first line.
Many thought he believed he couldn't do anything because he didn't want to do anything, hence the accusations of treason or sedition made against him.
If unilateral secession was unconstitutional, but no one in the federal government could do anything about it, that didn't leave the constitution with much authority.