Just as the Southern states wanted out of the empire completely. Empire.
...but the point isn't that hard to grasp.
You've done a great job making your point:)
You might say that rejection of the Corwin Amendment meant that Southerners who were active in politics thought something else more important than slavery. Someone else would respond that at the time they (wrongly) felt that slavery would be most secure outside the union rather than inside. It's possible that independence had become more important for secessionists than slavery at that particular moment, but what had motivated them up until that point? What was behind the movement? Slavery is a very good answer to those questions.
Slavery is a very good answer. But, it's incomplete. "Internal improvements" were a continual rub to those same Southerners and that brings us to the tariffs. When you include all of these things the picture becomes more complete. If slavery had been the sole motivation for the Southern states, they probably would have accepted the Corwin Amendment and moved on with their lives.
If you trust Northerners so little, and the secessionists of 1861 trusted them so little, maybe they didn't respond to the Corwin Amendment because they knew (or suspected) that Northerners weren't on the up-and-up, and wouldn't honor it.
Well, that's certainly one way to look at it, but I believe you had it right earlier in your post - it was simply too little, too late.
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If slavery had been the sole motivation for the Southern states, they probably would have accepted the Corwin Amendment and moved on with their lives.
Read the periodicals of the time, like DeBow's Review. Slaveowning elites were already focused on independence and expansion. When you can have your own empire, why be part of someone else's?
"That d-----d John Brown is trying to arm and free the slaves!"
"Is that all? I was worried he might be trying to build us a canal!"
"Heavens! At least it hasn't come to that yet!"
And yet looking at the statements of secessionist political leaders, the various declarations of causes, and so on, "internal improvements" hardly merit a mention and tariffs not much more. The threat that they perceived to slavery is all through them, though. The South Carolina appeal to the other southern states to join them in secession isn't addressed to the "Internal improvement opposing states" or the "Low tariff-favoring states." No, it's to the "Slave-holding States."
Corwin wasn't enough for the southern states--they wanted expansion into new territories, which is why Crittenden was proposed.