On Poland and Lithuania
There are currently Poles fighting in the Austro-Hungarian Polish Auxiliary Corps and in the Polska Siła Zbrojna under German command on the side of the Central Powers (the numbers vary greatly in different texts, I have no idea how many they were), and there are over 25,000 Poles in two Polish Corps fighting on the side of the Entente in the UoE as well as more than three times this number fighting in the "Blue Army" for the Entente on the Western front. And there are several thousand underground fighters in the paramilitary Polska Organizacja Wojskowa now mostly on German-occupied terrain. Politically, there is the Komitet Narodowy Polski in Paris, dominated by Dmowski's ND, and its affiliate Naczelna Rada Ludowa in Poznan, who are on the side of the Entente, closely aligned with France and with an ambivalent, but prevalently positive relation with *Russia. And the Rada Regencyjna, installed by the Germans in Warsaw under their loyal Prime Minister Jan Kanty Steczkowski, a very conservative and tame body obviously pursuing German-friendly policies, some probably out of conviction (they don't like what happens in the UoE), more probably out of opportunity. In Moscow, there's a Naczelny Polski Komitet Wojskowy, which is just as multi-party as the paramilitaries in Poland, but in contrast to the latter opportunistically committed to fighting along with the UoE on the Entente side. And then there are internationalistically minded Polish socialists scattered from Berlin to Moscow, operating outside these national-minded bodies. All much like OTL, except for the Polish Corps in Russia and the Naczelny Polski Komitet Wojskowy which IOTL were caught on the wrong foot by the October Revolution but which ITTL persist.
IOTL, Germany created (in some cases short-lived) facts on the ground in 1918 with Brest-Litowsk; also due to Brest-Litowsk, no single Polish force was still pro-Russian. With the Russian Civil War and especially its regional clusters, the Ukrainian Civil War and the Civil War in the Baltics, an ample window of opportunity arose for ambitious Polish politicians and military leaders to meddle in the East. It was in this climate that Pilsudski's vision of Miedzymorze developed and gained traction. With only half-hearted, tepid support from the Entente (because they fought against the Soviets, but they did not co-operate with the "White" Russian forces), the various factions soon coalesced at least militarily into one army on which one Second Polish Republic rested. ITTL, several circumstances have changed, creating a significantly difference context and atmosphere, with different opportunities.
As you have rightly pointed out, we do not know yet where the front lines will run at the moment of the armistice - even I don't know that yet. There is also the question, if the Germans have to retreat from the East in any form comparable to Compiegne terms for a retreat in the West, which troops on the UoE side are moving in to replace them. The Polish Corps and the Ukrainian Territorial Defense Forces will have fought side by side for well over a year, for example, but this is going to be a divisive issue for sure.
As for Lithuania, there is the Taryba in Vilnius, which, like IOTL, has both declared independence and declared "firm and special ties" with Germany in late 1917 / early 1918, but was not rewarded with much respect from OHL / Ober Ost. In contrast to OTL, as Germany does not look quite as strong in the East (even though they have taken Petrograd; they have not, for example, gained control over the Ukraine), there has not yet been an election of some Württemberg nobleman as King Mindaugus II. of Lithuania. Let us not forget, though, that many Lithuanians have fled Eastwards and live in the UoE now. Some of them have joined the Republican Guards. Resistance in Lithuania exists (the "men of the woods"), and in contrast to Poland it is quite unambiguously aimed against the German occupiers (and thus not exactly in sync with the Taryba's opportunistic policies), but also not such a big factor.
The new OHL and the new German government, who are willing to give a lot more leeway to their Central Eastern European puppets than before, are focusing on Poland because it's the much bigger factor, but they will soon recognise Lithuanian independence, too, especially when the UoE declines the armistice offer. The Taryba won't even have to elect a German king for that. This does create some facts on the ground which look similar to OTL, so Lithuania's joining the UoE is not guaranteed (remember, Chernov's blunder officially acquiesced to the Lithuanians "choosing their own path" once they're freed from German occupation). It might become an opportune option for the young Lithuanian Republic if Poland claims what they consider to be theirs - on the other hand, would a war-weary democratic Russia really go to war against Poland over Vilnius or Suwalki? Militarily, the Lithuanians are utterly helpless against the Poles who already have components for a national army at their disposal - if the different Polish factions can agree on a joint course of action. Politically, though, it's not a given that things won't be decided at the green table in *Versailles. ITTL, both the Western Entente and the UoE are at the table, and they will negotiate among themselves which representatives of Poland, Lithuania etc. they will accept as negotiating parties. There are considerably less loopholes (in the form of the geopolitical black hole the Soviet Union was IOTL) to circumvent any provisions agreed at the conference ITTL, and the conference will have to sort out the fate of Central Eastern Europe in much greater detail than IOTL precisely because, while allies during war, the UoE and, for example, the British are not likely to be political BFFs.
I fully agree on the fuzzy, essentially yet undefined boundaries of "Poland" and "Lithuania". The Germans will, over the next couple of weeks, probably try to initiate a straightening-out between their two puppet states, but all of that can be blown away by the German collapse all too soon. (Or it could outlive it, you never know.) What we should not carry over from OTL without questioning it, in my view, is the assumption of the great space of maneuvre in a political semi-void which existed IOTL and caused quick coagulations, "wars of the dwarves", and everything that followed from this nation-building...