Specifically, the wording of the Corwin Amendment was:
"No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State."
Upon a critical reading you will note that it forbade Federal interference in states with laws holding people to service.
Your comment that it did not legalize slavery everywhere was fallacious. Here is what you said: "In those states where slavery was illegal, it would remain illegal."
That is on its face not true. The amendment said no such thing.
In ratifying this Amendment, any prohibitive legal consequences for its practice were forbidden.
Thus it remained an option for each state to determine that issue themselves without fear of prohibition.
Should any state decide to reinstate slavery, as it could do so freely, then the Federal government could not interfere.
Simply put, if Indiana or Oregon decided by state law that wanted to reinstate slavery, then according to Corwin, there were no Federal regulations to deny that right.
More importantly, it is quite clear that Lincoln's alleged morality was mitigated by his need for political survival of the union.
"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.