This kind of political POD actually could be profoundly destabilizing in the Confederacy, particularly if the Union tries to play up the more Unionist elements who have no investment in slavery anyway to use this as a means akin to the 10% policy to worm into the Confederacy's political system from within. It would be a Xanatos Gambit at its finest as the CSA, no matter what it does here, benefits Lincoln. If it as per OTL expands the war in scale and horror then its leadership looks bloody-minded and deluded just as it did after repeatedly refusing Union peace overtures. If it reacts as it did IOTL to dissidents who see this as a means for a peace with honor, it shows its claims to any legitimacy other than the sword to be completely and utterly bunk. If it reacts to this with a Black and the Grey response then the CSA may survive but it will be so different from OTL that only the name of the state and its initials would be the same.
This, if it had happened, would have been a true political masterstroke.
Oh come on. Everyone already knew secession and the war were because of slavery. Compensated Emancipation in a few border states isn't really going to change anything. Unionists residing in the Confederacy will still be persecuted, OTL the Confederate leadership already looks bloody-minded and delusional, and prior to the war they had already repeatedly refused all peace offers. Anything this POD brings to the table is just a matter of small degrees. Perhaps it would curtail a few Confederate apologists in the post war era, but that's doubtful.
Sure it would be a feather in Lincoln's cap but its not a masterstroke.
Benjamin