Won’t agree to your stipulation. Lincoln’s objective was to restore the Union. Freeing the slaves in the States in rebellion was a very shrewd political move.
The original Constitution acknowledged the existence of slavery in the country. It did not mandate slavery. It did not require slavery. There was nothing in the Constitution that prevented the outlawing of slavery by any state in the Union.
Once war began, the enemy was not afforded any protection of the Constitution because they were the ones that repudiated the Constitution. They were the ones that voluntarily withdrew from the Republic that was established by that Constitution. Lincoln as CinC was allowed the leeway to destroy slavery in the States in rebellion. The Constitution no longer applied to any state in the Confederacy. Outlawing slavery in the States of Rebellion is not un Constitutional because the Confederacy was no longer subject to the Constitution by their own actions. He did not outlaw slavery in any state that remained loyal to the Constitution and the Republic.
"It being among my first wishes to see some plan adopted, by which slavery in this country may be abolished by law.” - George Washington Letter to John Mercer, September 9, 1786
“I wish from my soul that the legislature of this State could see a policy of a gradual Abolition of Slavery.” - George Washington: letter to Lawrence Lewis, August 4, 1797
“Every measure of prudence, therefore, ought to be assumed for the eventual total extirpation of slavery from the United States.... I have, throughout my whole life, held the practice of slavery in... abhorrence.” John Adams: letter to Evans, June 8, 1819
“It is much to be wished that slavery may be abolished. The honor of the States, as we as justice and humanity, in my opinion loudly call upon them to emancipate these unhappy people. To contend for our own liberty, and to deny that blessing to others, involves an inconsistency not to be excused.” -John Jay: to R. Lushington - March 15, 1786
“Would any one believe that I am master of slaves by my own purchase? I am drawn along by the general inconvenience of living without them. I will not — I cannot justify it, however culpable my conduct. I will so far pay my devoir to Virtue, as to own the excellence and rectitude of her precepts, and to lament my want of conformity to them. I believe a time will come when an opportunity will be afforded to abolish this lamentable evil. Everything we cam do, is to improve it, if It happens in our day; if not, let us transmit to our descendants, together with our slaves, a pity for their unhappy lot, and an abhorrence of Slavery. If we cannot reduce this wished-for reformation to practice, let us treat the unhappy victims with lenity. It is the furthest advancement we can make toward justice. It is a debt we owe to the purity of our religion, to show that it is at variance with that law which warrants Slavery.” -Patrick Henry letter to John Alsop Jan13, 1773
If that it true, and it very well may be, then we can forever dismiss the notion that Lincoln and the North fought for the high moral principle of “freeing the slaves.”
But the North did fight and for good reasons: it was in their economic and political best self-interest.