I don't think it's all that implausible to have Buchanan begin the civil war that much earlier... Take a look at his cabinet appointments, he definitely was going for a pro-Union, anti-secession feel. IMO All it would take is the right spark to get Buchanan to move (albeit reluctantly) against the Confederacy
A Potential Spark Perhaps?
Let's say that our POD is that the cadets from the Citadel score the proverbial "Golden BB" hit on the
Star of the West resulting in a catastrophic explosion which sinks the ship. The event is then sensationalized by the Press, and when South Carolina refuses to hand over the cadets to the Federal Government to be tried, Buchanan is forced to call up volunteers to suppress the rebellion.
However even his being a democrat I don't think would affect the decisions of the Border States or alter them from OTL. What you would see is a lot less war materiel falling into Confederate hands as they have 3-4 months less time than OTL.
The real question is how he decides to serve out the rest of his term. Does he attempt to do as much as possible to win the war to salvage his reputation? Or does he appoint Lincoln as his Vice-President and then resign viewing himself as a failure for not preventing the outbreak of hostilities.