My own answer to the question I poised is; No there would not have been a civil war.
Slavery intensified the differences between Northern and Southern Culture. The many irreconcilable differences between North and South had their origins in the use and spread of slavery. By the 1840s and beyond the growth of the telegraph, railroads, and western expansion had made the differences apparent to the common man of the United States.
And the choice of the majority was to avoid slave holding areas of the country. The first serious challenge to the south wasn't from the abolitionists but from the various free soil parties. The northerner saw the injustice imposed by the plantation owners not only on the slaves but also the poor white farmers of the regions. And they wanted no part of that.
Free Soil was the initial driver behind sectional conflict. Later this was coupled with a growing belief in that it was morally wrong to hold men in slavery. All of this came to a head in the 1850s when the South won several significant victories particularly Popular Sovereignty and Dred Scott. Fueled by those and other incidents like the Kansas' pro slavery Lecompton Consitution which was submitted by a rump conventation, the various anti-slavery groups of the north into the national Republican Party.
By the late 1850s both sides believed that victory by the other would be total and compromise became impossible. Dred Scott was the crucial moment when many believe the the Southern would use the federal government to get what they couldn't get by the vote of the people. In a single decision the Supreme Court nullified the individual state laws regarding the possession of slaves in their territories. The North viewed it only as a short step to allowing slave owners to live wherever they wanted with their "property" intact.
At that point the North united behind the Republican Party to ensure that the federal government would be used to their advantage and not the South. Understand the moderates only wanted slavery to stay in the South and out of the western territories.
But southern radicals were able to make an effective case that without the federal government dominated by southern interests (like it was under Pierce, and Buchanan) it would spell the eventual death of slavery and the ruin of millions of dollars worth of property. That the radical ablotionists will get the upper hand and turn federal power against the south ending slavery.
The southern radicals were able to win majorities (most of them slim) in the Deep South and started the process of secession.
Without slavery, there the differences between North and South would have not been great enough to cause civil war. The big political debates would been likely Jacksonian Populism versus Whig Industrialism, combined with debates over how to handle Manifest Destiny.