To summarize: the Civil war didn't start over chattel slavery. Secession was because of slavery, and the Union fought because secession was unacceptable. There certainly was an abolition movement, but it was small

To be fair, a wide swath of the Union (the midlands -see Colin Woodard's analysis of American colonization patterns https://medium.com/@colin_woodard) was perfectly willing to let them go - until the Confederate forces fired on Ft. Sumpter.

I can't help but wonder how things might have turned out differently. Would there have been a massive slave revolt like in Haiti and Jamaica - and what would the rump Union have done in response? Would the South turn into Sparta, every man militarized in order to keep the Helots in line? Would the Confederacy attacked Mexico or Cuba in order to gain more plantation land? Would they have eventually given up on chattel slavery?