I've read the same letters and speeches and while there can be no doubt of Lincoln's opposition to slavery, the question was still whether the rebellion was over slavery. The Union goal in that conflict, as stated over and again by Lincoln, was always the preservation of the Union itself. Given that, while the elimination of the institution was a fortuitous outcome of the conflict, it was not the goal of the Union war effort itself.
Why not hearken to the words of a confederate patriot - Robert Barnwell Rhett (nee Smith), who said: "Aye - disunion, rather into a thousand fragments. And why, gentlemen, would I prefer disunion to such a Government? Because under such a government I would be a slave - a fearful slave, ruled despotically by those who do not represent me, and whose sectional interests are not mine.... If to think, to speak, to feel such sentiments as these, constitute me a disunionist and a traitor ... then gentlemen, I am a Disunionist! - I am a Traitor!" Charleston Mercury, October 18, 1830