Thanks for making that excellent point.
It turns the arguments from FLT-bird and DiogenesLamp on their heads.
They claim Corwin proves secession was not about slavery since Confederates "rejected" the Corwin "offer".
In fact, as you point out here, it proves that secessionists would accept nothing less that full 100% legalization of slavery at all times in all places, which is what the new Confederate constitution gave them.
In contrast, Corwin still allowed for restrictions on slavery in territories and gradual state-by-state abolition.
Lincoln said he took that to be the old Constitution's meaning already and so did not oppose it.
But for secessionists the new Confederate constitution was a far better offer, so they took it.
My argument is that the Northern coalition cared so very little about the plight of slaves that they offered permanent protection for slavery in an effort to coax the Southern states into staying in/rejoining the Union.
That they offered it, and the Southern states were not moved by it, demonstrates clearly that neither side really gave a sh*t about the issue of slavery, and so modern attempts to portray the war as having been about that are just dishonest.
The one most damaged by the "Corwin Amendment" is Lincoln, who clearly offered his support for it in his first Inaugural address. It makes him appear to be two-faced.
100% legalization of slavery at all times in all places, which is what the new Confederate constitution gave them.
And effectively what the US constitution also gave them, though many people in the North refused to accept it.
Forbidding any state law to interfere with the return of a slave to his master effectively nullifies any and all laws against it. It was an ugly clause to the US Constitution, but the Northern states agreed to it, and so they should have been bound by it until such times as it was lawfully repealed.