You guys always try to spin the Corwin Amendment that way, but that's not what it would have done. It would have prevented the federal government from outlawing slavery, but it wouldn't have stopped the states from ending it on their own, as many northern states had already done. So unless you're going to claim that only the feds had the power to stop slavery, and that states had none, then the Corwin amendment merely restates what Lincoln had said over and over, including in the Greeley letter.
Now what Lincoln refused to support was the expansion of slavery into the territories. That's why he rejected the Crittenden Compromise, which would have done exactly the same thing as the Corwin Amendment, but would have also reestablished the Missouri Compromise line.
Here's Lincoln in a letter written in Dec. 1860:
On the territorial question, [2] I am inflexible, as you see my position in the book. On that, there is a difference between you and us; and it is the only substantial difference. You think slavery is right and ought to be extended; we think it is wrong and ought to be restricted.
Perhaps I misstated it. Thank you. The states always had the power to outlaw slavery. As you say, the amendment would have prevented the federal government from outlawing it.
I thought the Supreme Court expressed an opinion (or perhaps it was just Taney) that the Missouri Compromise was unconstitutional.
The treaty by which we obtained the Louisiana Territory said that residents could take their property (slaves were considered property at that time) anywhere within the Territory. If we abrogated the treaty, maybe we ought to give New Orleans back to France in compensation.