I've been sketching a TL where the CSA gains independence but doesn't do awfully well in its inaugural decade(s), leading to some of the states formerly holding high concentrations of Unionist sentiment wanting to quit the whole project.
We know from the proposed Constitution of the CSA that states anticipated the possibility of future attempts at secession (art. 1, sec. 8, cl. 15 retains the right of the federal CSA government to "call forth the militia … to suppress insurrections," which one could assume would include insurrections stemming from secession), and that, regardless of the legalities involved, it would not be a major leap to imagine the CSA, founded as it was in part on the principle that states should be able to walk away from the national government, facing secession issues of its own.
How might the USA react? What conditions might be imposed on those wishing to Confedexit prior to readmittance into the Union? How much does U.S. leadership & timing matter (i.e. 1880s vs 1870s vs almost-immediate-postwar)?
I'm mostly looking to use this TL to explore how some of the constitutional quirks of the CSA might have exacerbated the geopolitical and economic shortcomings inherent in the effort had independence actually been won, but I'm interested in gaining a sense of how plausible the ending is.