In alternative history genre there is something called "point of divergence" - that is, the point in which the history became alternative to what really happened. The further into the past such POD lies, the bigger the changes are. It's easy to understand if you compare it to, say, your first school or city in which you lived as a kid - would your life be the same if you went to a different school or lived in a different city? The answer is obviously no, it would be completely different in many ways, some more subtle than others.
And when the POD is hundreds of years into the past (counting from current year 1500 A.D.) the changes have to be proportionally greater. Dozens of generations lived and died differently, millions of people who lived in our timeline were never born and events like Great Dying, Sunset Invasion and others occured which fundamentally altered the history. So nope, there is no chance that the unique combination of factors which allowed for instance Copernicus to do what he did in our timeline may arise in "Imperium Sine Fine". This is madness indeed
That's true, yes, but that begs the question of how much is determined by individuals and how much
by broader social trends. Liberalism, for instance, was likely inevitable in some form or another- if you get industrialization and/or a burgeoning merchant class they are going to be agitating for a voice in the political system. And the kings helped this by essentially allying with the cities to crush the feudal lords. Republicanism on some form is also going to stick around with any PoD after the classical era- between Venice, Florence, the Netherlands, etc there are living oligarchic repblics and the Roman tradition to stand on.
England is one case of the push and pull between merchant, lord, and monarch. The PLC is another. But unlike England they were surrounded by Prussia, Sweden, Austria, the Ottomans, and Russia... There's a reason Poland is called the France of the east.
Germany had the problem of the Habsburgs, just strong enough to kick down Saxony and the Protestants, not strong enough to deter France and the Ottomans. Seriously f*** the Habsburg's, bloody inbred reactionaries thought they were CK2 players.
That said without New World colonization on the Anglican (and opposed to Hispanic) models and exposure to the much more egalitarian Iroquois/Haudenosothers) (among others) and George IIIs hamfistedness among others I don't think you will get the American brand of it.
Some sort of French/Russian revolution was likely also likely to happen in the autocratic states- while it was something of a perfect storm sheer chance suggests once you've opened the liberal genie all you need is a war, a famine, and an incompetent/indecisive monarch for it to explode... Or you could have gotten more Prussian/Sardinian style nationalist libera
lism,attempting to harness the agitation for territorial gain (though Napoleons role here cannot easi ly be overstated, I think some sort of "Germany" and "Italy" are likely to emerge somehow, though they could be radically different e.g. a Lombard kingdom of Italy sans Venice and Naples/Umbria or a unified HRE kingdom including Switzerland and the Lowlands...)
In the context of This Is madness,, you are somewhat justifed- there are no real "nations" here in Europe though China counts. With a lingering universalist empire based around religion (more like the Ottomans, Safavids, and Mughals to an extent; also China if you factor in ethnicity/culture) regional "nationalisms" will be suppressed some but I think centrifugal tendencies will still make it pop up..
Historically a lot of nationalisms were born in reaction to empires fighting over them/creating buffer States, e.g. the Netherlands as reaction to Habsburg tyranny, Ukraine contra Russia and the PLC, Finland between Sweden and Russia etc. So Italy for example might well cause trouble late game if and when the Aztecs are weakened, based around some regional identity. Egypt also around the Copts. The rest of Europe will be wierd demographically given the rather ahistorical (excepting the treatment given the native Americans...) mass extinction and resettlement... Total extermination really wasn't overly practical before the modern era though population transfers to create local cadres of hired goons was used a lot.
Tis a fascinating subject.