What if Poland and Lithuania never unified?
Well, that's a question that is rather not so common abroad when it comes to alternative history. In my opinion, we would simply have a continuation of the previous Piast policy. That is, strongly simplifying such France/England of the East. Poland would have remained constantly focused on the west rather than the east. We would have had Polish intervention in the Thirty Years' War, attempts to take back Silesia and New March. Subjugation of Pomerania and seizure of lands up to the Elbe. Possibly even attempts to conquer Bohemia.
How does each country likely develop differently internally over the next 400 years from 1385 or so?
Poland Proper may be even richer than it was OLT and this is due to the lack of Lithuanian lands by which investments that went there will go towards Polish lands. Lithuania will be poorer, the lack of a longer union is also the lack of Polish influence in the long run that expanded the country.
In terms of distinctiveness in terms of hereditary versus elective monarchy and balance of powers between the King/Grand Duke and estates/nobility?
Well that's a tough question, but one might be tempted to say that this ATL Poland would be an autocratic constitutional monarchy rather than a parliamentary one, but certain features of a republic were inevitable anyway. (There will be something called a mixed monarchy).
From what I conclude? The absence of the Jagiellons and thus the reliance on the magnates when it comes to power. Such an alternative ruler, (let's simplify that it is a Piast from the Mazovian branch of the family, let's call him Siemowit) due to the fact that he has no experience of ruling over Ruthenia will prefer a more classical approach of relying on the middle class. And this middle class supported the execution movement, the lack of Lithuanian miscreants from the upper classes, equals less resistance. The result should be a much more centralized Poland in which, although the Sejm is there, it is not decisive in everything.
Lithuania, on the other hand, looking at what was OTL, will most likely evolve into an oligarchy. Unlike ATL Poland where the nobility will be equal in rights and duties regardless of position, so here the lack of Polish influence means that social relations in Lithuania will be far less egalitarian, closer in this to the old Ruthenia.
How does each country's standing and position and strength and size in central and Eastern Europe likely evolve in the 400 years after 1385?
Well, Poland is likely to be a major power in the region, just like OTL. The foundations that allowed it to become this OTL were already laid by Casimir III, it's just that Poland will not be as great as OTL was. At the same time, due to the lack of a union, it will not get involved in eastern disputes. Thus, there will be no wars with Moscow, the Tatar invasions will be more of a Lithuanian problem than a Polish one. And that energy that was expended on wars in the east will be used for wars in the west.
Although certainly the map will differ significantly from what is known, so this is rather strong theorizing. Possible are, for example, a Polish-Moscow alliance through which Poland occupies the rest of Halich Ruthenia aka Red Ruthenia.
Poland may attempt eastern conquests using the title of King of Ruthenia and thus try to snatch Rus territories from Lithuania. One thing is certain, Lithuania will have considerable problems which it will have to solve on its own.
Do either of them become completely extinct as a state or language by 1785, or far earlier?
Lithuanian, they will succumb to Russification, which was also happening OTL, only the fact that Polishness was even more attractive made this trend stop and go in the Polish direction. Without Poland, it is even possible that Lithuanians will become Russified.
How is the fate of the Jewish communities who lived there, grew there, and settled there altered by the starkly different politics?
They will live in Poland, although it is possible that some King will decide that there is a need to make room for the Polish bourgeoisie and drive them out of the country. But rather they will stay and only smaller groups will go east.
What about different impacts of the Reformation
Few, though possible with the right ruler, is the rise of the Polish National Church. (Although this is doubtful looking at the OTL as well as the fact that Poland was under the Pope as sovereign not to have a German sovereign in the form of the Kaiser.
the borderline with Orthodox Christianity?
There is no Unician alias Greek Catholic church. Possible displacement of Orthodoxy from the territories of Polish Ruthenia.
Could Poland simply directly absorb Lithuania in this TL?
Possibly, under the casus belli of the King of Rus but it is not certain. Rather, it is more likely to take some pieces out of Lithuania.
I think Lithuania was the much stronger partner.
That's the wrong thinking, from the very beginning Poland was the stronger partner. Jagwiga's age did not matter much. She stood at the head of a more powerful and organized kingdom. Lithuania, on the other hand, was only big on the map, but in reality her power in many regions was weak and the ambitions of the Jagiellon family and their cousins got in each other's way. A good example is Jagiello's dispute with his brother Vitold.
