He hints it is illegal more than taking a hard line.
“It is said, however, that the antecedents of the President-elect have been sufficient to justify the fears of the South that he will attempt to invade their constitutional rights. But are such apprehensions of contingent danger in the future sufficient to justify the immediate destruction of the noblest system of government ever devised by mortals?”
Buchanan never answers the question directly. But he does hint that he doesn't think the time is yet ripe for immediate dissolution.
Your argument - Buchanan was hardline in a soft way - seems skinny if not lightweight.
It seems clear that he did, maybe not to your satisfaction or the secessionists', but he did answer the question in a way that left no doubt as to his own opinion.
And this brings me to observe that the election of any one of our fellow-citizens to the office of President does not of itself afford just cause for dissolving the Union.
.....
It is said, however, that the antecedents of the President-elect have been sufficient to justify the fears of the South that he will attempt to invade their constitutional rights. But are such apprehensions of contingent danger in the future sufficient to justify the immediate destruction of the noblest system of government ever devised by mortals? From the very nature of his office and its high responsibilities he must necessarily be conservative. The stern duty of administering the vast and complicated concerns of this Government affords in itself a guaranty that he will not attempt any violation of a clear constitutional right.
After all, he is no more than the chief executive officer of the Government. His province is not to make but to execute the laws.