I have been casually looking for information on the South's interest in and/or support for possibly building a southern railroad route across Texas and the southwest. I feel this could shed light on the CSA’s positions as to expansion. I haven't as yet found much info.
I would encourage you to look further south. The maps presented here seem to end at the Rio Grande, as if that was the edge of the world. Jeff Davis and his Confederacy had their eyes on a much bigger market for their peculiar institution. Some might call it a golden circle that stretched through Mexico, down through Latin America, across northern South America and all the way to Cuba. They didn’t need any Corwin Amendment, or even western expansion. They had their eyes on a much bigger picture than some here (with their blinders on) discuss. It’s a waste of time even discussing the Corwin Amendment and or expansion into the “territories”. The Confederacy had high hopes to become a main player on the world stage separate from the parasitic Northern States.