Lincoln endorsed it in an attempt to bring back the seceded states in the period before the firing on Sumpter. He believed that even though he opposed slavery he as president did not have the power to abolish slavery. He hoped to keep the union and the slave states intact while limiting slavery to just those states where it already existed. He also believed that slavery in the remaining slave states would eventually fall or that a 13th amendment would be passed.
The southern secessionist democrats chose to continue on the path of secession and war which led Lincoln to reassess his views on his presidential powers. After secession and Sumpter the confederacy no longer considered themselves under the Constitution and were at war with the Union. As long as he won, he could use his presidential proclamation to emancipate the southern slaves in secessionist states in rebellion against the Constitution.
For the south, it was all about slavery, period. For the north in general and Lincoln it was about restoring the union first and slavery could be resolved in a non-hostile environment. As the war progressed, the abolitionist sentiment increased in importance.
If all the Confederacy desired was the perpetuation of slavery, all they had to do was to remain in the union.
With the Dred Scott case intact, the Corwin Amendment sent out for ratification, and Lincoln's endorsement...which meant that there was no governing authority to threaten slavery, then remaining in the Union meant less risk.
The facts contradict your conclusion.
To correct your thinking, it was secession that created the new government, and of course the seceded states were no longer under Union authority.
That was not the South being “at war”; it was simply secession.
The war started in Lincoln's office when he initiated the blockade and thus brought war to the South.
Not true.