The proper course for history would have been for our Founding Fathers not to found. The compromises worked into the Constitution to accommodate the slave states essentially kicked the can down the road, and all but insured the Civil War.
I believe, had two nations been formed, or none, or a northern one and a handful of separate, sovereign states/countries in the south, that the forward course would have been better for the people of this continent.
And yes, the southern states would eventually have ended slavery, as did all European-derives countries. The slave trade had been going on for many centuries run by Moslems and Black Africans before any Europeans got involved, and it went on (run by those same people) for many centuries after European people ended it, having found it incompatible with their Christianity and their basic ideas of fundamental individual liberty.
Your scenario would/could never have succeeded. The Brits would have consumed us piecemeal. The different factions north and south were aware of their respective cultural and ideological differences and signed up regardless because the alternative was simply unthinkable.