With the bolded part, I was actually thinking less of slavery and more of coerced labor in general. You are absolutely right that slavery was ended before we saw a modern industrialised economy, but things like sweatshops and some of the things done to migrant farm workers exist and are profitable. While they certainly aren't slavery, they are forms of coerced labor, at least to some degree and they work, as do indentured labor, peonage, debt slavery, wage slavery, corvee labor, and straight up impressment - so I don't see why slavery couldn't work in the same roles in TTL. This is in addition to the sad truth that slavery is far from gone in the present day. It is alive and well in many parts of the world, just less well known and thankfully less widespread than it used to be.
That's interesting, but does make a certain amount of sense. Do you happen to know why slavery was less efficient per capita?
I guess, I'd disagree because I see the CSA as far more likely to manumit and maintain their system largely intact with some other name serving as a fig leaf for slavery than refuse and see the whole system jeopardized