I finally was able to read through the entirety of the Decades of Darkness in the past few weeks, though I knew about the broad outline of the TL and had skimmed significant portions of it before. It is truly a monumental work and one of the most elaborate fictional universes ever created.
Some questions and speculations:
1) We see glimpses of the *American and German film industries, but what of the animation industry? Will this TL have the equivalent of a Walt Disney who not only pioneers animation techniques but creates multiple classic films and characters? Given the DOD US is much more relaxed about censorship cartoons may be seen as less of a strictly kids thing in TTL, with there being more edgier cartoons with humour aimed at adults. Alternatively, it may be Germany that pioneers more child friendly animation (Germany after all being the home of Grimm's fairy tales) while US cartoons are seen as more adult oriented. Something like anime (although obviously with very different styles and tropes) with serious mature storylines and high production values along the lines of
Akira or at least Studio Ghibli films may eventually emerge in the United States.
Incidentally given DOD US identifies so strongly with Rome, a version of
Hail, Caesar! set in Hanseltown would be very amusing to seen. The US/Roman identification may produce anti-Roman epics in the other powers. For the most obvious example, Germany can make a film about Arminius and the Battle of Teutoburg Forest where Arminius is portrayed as sort of a proto-Eunuco Mitchell dedicated to protecting his people from aggressive slavers. A film about Spartacus seems highly inevitable as well.
2) How absolute is the First Amendment in the DOD USA? You do mention that there are reformists and closet abolitionists who work to make the system more humane and refuse to personally own slaves such as Cordell Hull and advocates of slave rebellion probably could be prosecuted under "incitement" grounds. However, are there radical abolitionist organizations and individuals who urge for gradual and peaceful but full emancipation of all individuals in bondage and perhaps even granting them the rights of citizenship? Are such groups allowed freedom of speech and assembly? Certainly, they will be very well outside of the mainstream and it would about as socially acceptable to be an abolitionist as it would have been to be an open communist in OTL 1950s USA. They would probably be blacklisted from the major professions and the universities as well as be unelectable to political office but I would hope that those few lonely voices are permitted to exist.
3) Given you mention that the President of the *United States in 1949 is a historical figure and he's mentioned as pursuing a "Good Neighbour" policy, is the US President Cordell Hull? Does Nielsen only become President in the 1950s?
4) In
the Fox and the Jackal, why did Leydon's childhood friend Annabelle flee from New England to South Africa? Did this occur during the Vitalist dictatorship or Mullins's later overthrow?
5) What does the New England party system look like under the Commonwealth? I suspect the Socialist Alliance may be in for a long period of dominance similar to the Swedish Social Democrats OTL. I could see a lot of the more left wing Radicals joining the Socialists to create a big-tent left party while also simultaneously moderating the Socialist agenda. The opposition will be an odd coalition of more moderate Radicals, Federalists, and even ex-Vitalists. Its somewhat hard to see them work together after the Mullins dictatorship, so perhaps the Right will split into a centrist/center-right pro-business Radical Republican Party and a post-Vitalist National Party. Its possible the New England government will place restrictions on explicitly Vitalist parties but given Mullins was overthrown by coup there could very well be a hardcore of continuing Vitalist supporters of around 20-25% of population. Wonder what happens to other high ranking Vitalist figures such as Rundle and Ingersoll.
6) How common is capital punishment in DOD? I certainly don't see the United States abolishing it anytime soon and if anything, given the Southern dominance of the US, it will probably be even more commonly used than in OTL 1940s and 1950s US. A few of the northern states such as Iowa and Wilkinson may never have allowed it as was the case OTL in those states. No idea how commonly capital punishment was used in Vitalist New England but its very likely that it ends up abolished under the Commonwealth, at least after the sentences for Vitalist era figures have been passed. I suspect Germany will retain it for much longer than OTL without there being a reaction to Nazism.
7) How big is the American Socialist Party? They are not mentioned as being present at the Third Socialist International in the 1880s but it seems likely to me that there is at least a small Socialist Party consisting of recent immigrants, some labour union types, and more leftier ex-Populists. Incidentally how strong are labour unions in the DOD US and do they tend to favour the Unionists or Democrats?
8) Some thoughts regarding urban demographics in New England and the US:
-You mention New York City being even larger TTL than in OTL but I'm not sure if that will be the case. NYC isn't the megapolitan cultural, financial, and immigrant center of the United States like it was and is OTL, even if New England gets a disproportionate share of immigration to North America. In particular, the Jewish population of NYC will be rather smaller than OTL's population at mid century given the substantially improved conditions for Jews in continental Europe.
-Given the more southerly orientation of the US, I expect the cities of the Mississippi and Ohio River Valleys to be much more populous here. In particular, Louisville and Cincinnati may be slowly merging into each other to create a huge conurbation. However, given the huge utility of the Great Lakes, cities like Erie and Cleveland will be pretty significant despite being on the border of New England. Ditto with Pittsburgh given its a very logical site for the steel industry.
-Cities along the Gulf Coast will generally be larger given its status as an American lake in this world. Pensacola will be a massive military city and *Houston (dunno what TTL name is) will be booming.