I too have not read Decades of Darkness so i cannot comment.
Another potential issue could be to make Slavery more of a divisive issue, earlier than in OTL.
In OTL, even in the southern states, it was tacitly accepted that Slavery was on its way out-this is why there was relatively little outcry when the US agreed to a treaty in the early days, banning the importation of slaves.
It was only the invention of the Cotton Gin by Eli Whitney in the 1790s, that Slavery became economically viable again-and from there, the tensions that ultimately resulted in the civil war started to grow, exacerbated by the growth of the union and the addition of new states.
If you can have a POD being simply the earlier invention of the Cotton Gin. then you potentially bring forward the emergence of North-South tensions over slavery.