OK, let's try a different tack. Forget Brazil.[And Cuba] Two scenarios; one where there is a CSA victory, however improbable. So they win, Garrison, et al. seethe with impotent fury, and slavery marches on. But the rest of the world, bit by bit, starts to regard the CSA as something akin to apartheid SA. Now, I would have been ashamed to live in such a state, with the rest of the world condemning me. So, attitudes begin to change, first, in the upper south. How much agony can be involved in going from de jure slavery, to de facto slavery, which is what sharecropping was. Maybe the plantation owners see their profits drop 10 -15% or whatever, but their workforce, for the most part, has to stay; plus, you can have all the Jim Crow laws you want, segregation on steroids; no one is talking equality for the races here. And this is so much better than being treated like the world's leper colony. When the Upper South goes through this transition comparatively painlessly, from about 1870 - 1880, then the rest of the CSA sees the benefit of it, and eventually follows suit, circa 1890. Yeah, yeah, I know, the sacred CSA constitution; so the state govt of Tennessee votes to phase out slavery anyway, and says F you, Richmond, and what are they gonna do, start another war?
The next scenario involves no civil war, again borderline ASB perhaps, but whatever. You need 75% of the states to ratify a change in the constitution? Let's do the math. Say by 1880 it is attempted. Now, no war means WV is butterflied away. Maybe even NV. [There was a political reason for Nevada's early entry into the union, I don't remember what.] So with these 2 not in the mix, I count 25 states in the OTL USA and 11 traditional CSA ones. Not yet enough, but if you count the 4 upper south states as being ready for abolition by now, then it goes to 29 vs 7, which is enough. Spring forward to 1890, then you have the 2 Dakotas, Montana, Washington, Wyoming and Idaho, by now its pretty much a slam dunk. Voila, bye bye slavery, welcome to sharecropping. And if I can't convince you with all that; that there will be no 20th century slavery anywhere in America, no way, no how, then I respectfully give up.