The only real question about Buchanan is and should always be: "Did America have a secret traitor President or was he just so dumb that he didn't see his Secretary of War committing treason?"
I don't think Buchanan was a traitor or dumb. I think he underestimated the lengths that secessionists were ready to go towards the end of his term, particularly after Lincoln's election, and he was paralyzed by either fear or resignation that he was powerless to do anything to stop secession. It doesn't excuse him, though. Buchanan didn't want to start the war by making it look like he was taking action that could trigger hostilities. But his inaction allowed the embryonic Confederacy to build up early advantages that the South never should have been able to have once the war did start.
It is crazy to me that Buchanan's Secretary of War that you referenced -- John B. Floyd -- isn't as well-known as Benedict Arnold because Floyd was arguably an even bigger traitor. All of the Confederate soldiers were traitors, but Floyd should be remembered as one of America's biggest villains. He committed Treason (with a capital 'T') while he was a still not only a member of Buchanan's Cabinet, but the civilian leader second in command authority of the U.S. military behind only the President.
Ulysses S. Grant made sure to remind everybody of Floyd's treachery several times throughout his Personal Memoirs of U.S. Grant (BOOK | KINDLE), suggesting in his section about the Battle of Fort Donelson where Floyd commanded the opposing Confederate forces, that Floyd was particularly anxious about being captured by Union troops when the Confederates were about to surrender (Floyd gave up command before the formal surrender and fled):
"General Floyd, the commanding officer, who was a man of talent enough for any civil position, was no soldier and, possibly, did not possess the elements of one. He was further unfitted for command, for the reason that his conscience must have troubled him and made him afraid. As Secretary of War he had taken a solemn oath to maintain the Constitution of the United States and to uphold the same against all its enemies. He had betrayed that trust.
As Secretary of War he was reported through the northern press to have scattered the little army the country had so that the most of it could be picked up in detail when secession occurred. About a year before leaving the Cabinet he had removed arms from northern to southern arsenals. He continued in the Cabinet of President Buchanan until about the 1st of January, 1861, while he was working vigilantly for the establishment of a confederacy made out of United States territory. Well may he have been afraid to fall into the hands of National troops. He would no doubt have been tried for misappropriating public property, if not for treason, had he been captured."
