Fascinating update, and very well written.
Wild speculation time!
We hear a little about a Concordat, presumably a political alliance between Burdenist elements (or the government?) of Carolina and the Cherokee Empire (and potentially other Indian states allied with the Cherokee). Maybe this is what the Great American War will be: not simply a civil war between pro and anti-slavery elements in the ENA, but an alliance between the Superior Republic, Carolina, and various native states against the northern Confederations and the Empire of New Spain. The core of such a war would seem to be Burdenist in nature, with the riot in Spain being a catalyst for a new perspective within that ideology that Spaniards (or Europeans?) are racially inclined towards imperialism and authoritarianism, and must be driven out of the Americas. At some point, the UPSA and its Andean Allies will claim a similar perspective and enter against New Spain.
Now, whether the Meridians are on the winning or the losing side, such a war could definitely yield an adoption of Societist ideals, or at the very least provide a fertile political landscape rife with convergent mindsets. If the UPSA wins, it may take steps towards de-emphasizing national identity as a means to consolidate territorial gains. If it wins, but has a falling-out with the "Burdenist Alliance" in the process, or soon afterwards, a disillusionment with nationalist ideas may sweep the political spectrum.
The most interesting scenario is if the Meridians are on the losing side of this hypothesized Great American War. Considering the ENA's naval capabilities, the UPSA may suffer severe blockades and sanctions, which will encourage the most powerful elements of society (political elite, landowners, plutocrats, aristocracy among the indigenous nations) to take political control and extend influence. This trend may prove to be universal, across the lines of ancestry and religion (example: British aristocrats using family capital to start a new life in the region after being driven out of their traditional land), and we see proto-Societist ideas develop as a political justification for the aristocratic rule.
Also possible is that the UPSA's society effectively crumbles either from war stress or a complete defeat (I'm drawing a parallel to OTL's War of the Triple Alliance, where Paraguay basically had to start from scratch but continued as a nation). Adventurous aristocrats fleeing liberal Europe find a place here, as do political elements of the ENS. Similar to the first "UPSA loses" scenario, except that these aristocratic elements were never members of the same nation before ruling it together.
