Having already announced their secession and adopted a constitution that protected slavery to a far greater extent than the Corwin amendment ever would have, do you honestly expect that the southern states would then have called it off?
Clearly something other than concerns over slavery must have been motivating them.
Apparently not, if the writings of the southern leaders of the time is to be believed.
And yet there is no escaping from the fact that the Northern states were willing to protect slavery forever in the Constitution itself and the original 7 seceding states turned this offer down.
Clearly they were far more concerned with issues other than slavery. The leaders of those states made it quite clear they had huge concerns about economic issues (ie the tariff and unequal federal government outlays) that would not have been rectified by the Corwin Amendment.