I think you have read Lincoln and the Radicals dead wrong on that issue. I've read Lincoln's letters and speeches from 1855-6, and there cannot be the shadow of a doubt that he fully intended to terminate slavery, with or without the consent of the Southern States. Certainly Southerners were correct in their estimation of Lincoln's devotion to abolition, judging by outcomes.
The "fortunate outcome" of the war was neither "fortunate" nor, to use the more perspicuous word, fortuitous.
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.