For this to happen, it's almost ASB. Buchanan, while a unionist in the broadest sense of the word, did not see the Union as sacred as did Lincoln and the other followers of Clay's American System ideology. Instead Buchanan placed the Southern view of "states rights" as superior to the Unionist cause. This essentially boiled down to...the nation as a whole bowed the will of the individual states versus the states bowed to the will of the nation. Of course Buchanan and the South always included the caveat that everyone bowed to the will of the slaveocrats.
While he may have been bullied by Northern Democrats into sending Marines and Regulars to numerous federal forts throughout the South, it is nearly impossible to rationalize his sending Northern militia or making active moves to surpress the militia. The idea of secession already had a large and dedicated following in the South, and Buchanan though a Southern sympathizer was not trusted as he was from the North. So really once Lincoln was elected secession was imminent. Any half hearted effort made by Buchanan would have only made Virginia, North Carolina and other Upper South states secede earlier.
I don't like Buchanan at all (I would list him as one of the four worst presidents) but had he attempted to half-heartedly surpress South Carolina it's possible he could done such damage that even KY, MO and MD would have bolted.
Benjamin