What an interesting issue. Of course you are correct, and early in the war Lincoln did say
if he had to keep slavery to preserve the union, he'd do it, and if he had to abolish slavery to preserve the union he'd do it.
I reckon my point is that, IMHO, Lincoln was always an Abolitionist (of sorts) and that as the War Between the States ground on, he came to see more clearly that Slavery and the Union could not legitimately co-exist.
I am also well aware of the effort toward "Gradual Emancipation with Compensation," "Colonization in Africa," "Colonization in Nicaragua," and all the other trial balloons Lincoln sent aloft, and the general racial views he held at one time or another. To my mind, that he became a fervent abolitionist does him credit.
I agree and disagree. I think he was an abolitionist, but he was also a railroad lawyer -- that was his profession. He worked patent cases for businesses making machinery, and railroad cases. The fact that he was a railroad lawyer has never been given the weight it deserves, I think.
Now, was he an abolitionist as a grand-tactical maneuver, to unite the North against the South, seize the government, and deliver to the railroads and the manufacturing Interests the Nirvana of business-oriented, top-down, centralized government that the United States gradually became, from the Gilded Age onward? (At least until FDR was elected.)
Was he an ideological abolitionist looking for a way around the Constitution? A letter he wrote in 1855, which was analyzed by David Donald in his 1999 biography, Lincoln, suggests strongly that that was the case. War would let him abolish slavery, because war would suspend the normal processes of government and elevate the presidency to nearly Napoleonic powers. He could suppress the Southern politicians and reorganize their States as provinces, protectorates, territories, or whatever -- and meanwhile use that politico-military domination of the South to pass amendments to the Constitution that would wash clean his usurpations and carve his will in granite.
Lincoln's most salient personal characteristic was opacity -- secretiveness in his purposes. So we must always speculate, since he didn't really open up with anyone, ever.