Even if Lincoln had accepted session, or the South had won, by 1900 at very latest slavery would have ended in the states of the Confederacy. Would the bondsman have suffered more under slavery between 1865-1900 than he did under Jim Crow? Would his posterity have been better served by a South that initiated emancipation or one that had it forced on them from the outside? I don't claim to know the answers, but the questions are not fatuous.
Also...There is the question of the massive growth of federal power after the Civil War. Our nation was never again referred to as “The United States **are** ....”. It became, “The United States **is**...”
No, those questions are not fatuous. But there are others. For discussion sake, let's say Lincoln saw fit to allow the southern states to secede, and the Confederacy become it's own nation. There could then be the possibility that the lessened Union could then vote for the repeal of that portion of the Kansas/Nebraska act that repealed the Missouri Compromise of 1820, and outlaw the issue of slavery in those states. They could also pass a law that slavery would not be allowed in any further territories or states.
What then would the South do? If they continued with slavery in their initial, few states, it would begin a fairly increasing death cycle. Their only out would be westward expansion, or expansion elsewhere (Mexico, Cuba...). But expansion within what's now the continental US would likely be opposed by a more solidified Union. Would that then make the war just as inevitable as it was in 1860?
All is conjecture. Who knows how history would have unfolded. But the issue of slavery was a cancer within this country from before its beginning. Is it possible it could have been cut out without a major upheaval of blood? To me, that's the major question.