"It would legalize slavery everywhere in the Union." That is not true.
The Corwin Amendment would not legalize slavery everywhere in the union. Only the states, acting on their own, could do so, which of course they could do anyway before the passing of Corwin.
The only purpose of Corwin was to re-ensure slave holding states that the Feds wouldn't come in and free their slaves.
Lincoln supported it because it made no material change to facts on the ground and yes, Lincoln desparately wanted to avoid a war.
1. The Dred Scott Decision from March of 1857 clearly showed that the Supreme Court supported slave ownership.
2. With Lincoln's behind the scenes maneuvering, it is clear that he was not supporting abolition.
3. With Congress’ adoption of Corwin, it formalized protection of states from congressional interference with slavery, and made no limitation on state sovereignty regarding its legality.
But it also can be considered to be an amendment to Article V of the Constitution, because it would significantly and expressly have curtailed future federal amending power.
As originally ratified, Article V included the express limitation on prevention of the abolition of slavery by amendment until 1808.
The Corwin amendment in effect renewed the limitation in perpetuity.
You would agree that this meets the definition of legalization of slavery.
Perhaps more importantly, in summary, on the eve of Lincoln sending ships to Charleston and Pensacola to initiate the conflict, all three branches of the United States government were endorsing and protecting slavery.
That is lost on people such as Ken Burns and Spielberg.