While you wait for fresh answers by qualified experts, you may be interested in some previous responses on this topic.

This thread contains a few great responses specific to Lincoln's motivations and beliefs.

In particular, /u/sowser's response, found here.

With a relevant excerpt

In the case of the push for emancipation and abolition itself, it is true that Lincoln went into the Civil War with the overarching goal of keeping the Union intact at any cost; the abolition of slavery only becomes a clear and definite war goal later in the conflict as Lincoln, his administration and his party grow increasingly convinced of both the desirability and the plausibility of securing total and immediate emancipation.

And /u/freedmenspatrol's response, found here.