The TL that I'm working on currently has the CSA giving up slaves by 1900 via industrialization. I'm trying to make it (or force it) to be as realistic as possible and not ASB at all. I've got a map, and the idea for a flag that works off of combining the Bonnie Blue and the Third National.
I don't know about widespread emmancipation by 1875, I have doubts about it.
However, if you wait until the Boll Weevil hits the CSA much like it did OTL's South in the 1890s (possibly earlier than OTL given butterflies) it is concievable for several important CS states to have dropped slavery by that time on an individual state-by-state basis, such a thing will only be sped up by the destruction of the Cotton economy, which slavery was dependent on.
If I had to put a timetable to how slavery ends in the CSA I see it like this:
1880s/1890s: Boll Weevil crosses into Texas.
1890s/Turn of the Century: Upper South states abolish slavery.
1900s/1910s: A few Deep South states (Louisiana, Alabama, Florida and Georgia perhaps) abolish slavery.
1910s/1920s: South Carolina and Mississippi are the only remaining CS slave states, and economically they aren't doing well. Something could prompt them to abolish it as well.