I do not believe the "South" was united on secession
On this one point you are correct. North Carolina and Tennessee certainly were in no hurry to leave the Union as neither was Arkansas and Virginia. Checking the secession convention votes would give one a very big clue in that regard.
His party was a sectional entity, practicing their form of ideology exclusively in Northern states in 1860.
Perusing the situation show me that there were four sectional parties by 1860...Northern Democrats, still trying popular sovereignty...Southern Democrats, demanding to take their "property" to any territory they chose...Republicans trying to limit slavery to those States it was already established in and the Constitutional Union Party, composed of old-line Whigs and Know Nothings whose platform was rooted in the illusion the Union could be saved by ignoring slavery altogether. All four of these (now) sectional groups preaching to the choir.