Stephens was a moderate on the issue of secession. He was, in fact, friends with Lincoln. He argued against secession in the Georgia convention elected to deal with the issue and was elected Vice President of the CSA for that reason. He was no “fire-eater” on the issue of separation. That was my point.
As to slavery, certainly the issue was important to the South, but to say that slavery would or would not have continued had the South won the war is pure conjecture. There were those, Robert E. Lee for example, who were more moderate on the issue. Perhaps the South would have gone the way of Brazil, which ended slavery in 1881. Perhaps not. Slavery was profitable in the cotton producing areas of the South, but less so in the upper South. This may have led to some reform on the issue. Of course, the Confederate Constitution strictly forbade the central government from interfering with slavery in the States. It would have required a constitutional amendment, just as in the U.S. Stephens, of course, backtracked on this issue after the War, which has led some to suggest that he was disingenuous in his history of secession.
As Kevin said, however, the issue of secession (self-determination) and the end of slavery are two separate issues. The South certainly had the right to secede, just as the American States seceded from the British empire in 1776. To suggest otherwise is to reject first American principles.