“In 1861 Ohio Representative Thomas Corwin proposed an amendment to prevent Congress from interfering with slavery in any state. It would have been the thirteenth amendment to the Constitution. Congress approved it, but eleven southern states seceded from the Union before it could be ratified.”
Sounds like it was proposed after minds had already been made up.
The salient point is that the Northern dominated congress, controlled by Republicans voted for this permanent slavery amendment by a 2/3rds majority.
What this demonstrates is that the North and the Union did not care about the issue of slavery.
They did not invade the South to interfere with slavery, they only wanted to stop the South from having the Independence to manage their own economic affairs.