Yeah, I don't see any way interwar Ukraine could survive in this White victory scenario. The bigwigs - Kolchak, Denikin, etc. - were staunch 'Great Russian' nationalists, as I said earlier Kolchak's slogan as Supreme Ruler of the Whites historically was literally 'One and Indivisible Russia', and Ukraine is the 'Little Russia' in 'All the (Indivisible) Russias'. Having to cede Poland, Finland et al. on paper must already have been as painful as pulling teeth for those guys, there's no way they'd relent on any of the actual 'Russias' in 'All the Russias' if they still had any strength (and hope for legitimacy). Also quite a few White officers and their families hailed from territories claimed by the UPR as well: Drozdovsky for example was born in Kiev,
and Kolchak's own Romanian ancestors were from Bessarabia. Apparently Lavr Kornilov, the second great Southern White Russian chief between Alekseyev and Denikin,
was an exception and the only major White commander willing to entertain the prospect of Ukrainian independence, but nobody else agreed with him and he's already been dead for ~6 months before the massacre at Ipatiev House anyway.
Meanwhile as you say, the Ukrainian People's Republic was really weak and unstable. I don't think the Ukrainian army actually won a single major battle on its own against any opponents in this period. Even Pavlo Skoropadsky, the pro-German hetman, was a Russophile first and only became a German collaborator because it looked like Russia was done for & a Bolshevik takeover seemed imminent; if the Whites are ascendant he'll almost certainly just rejoin them,
he actually did try to work with the Volunteer Army around the end of WW1 after all. So they have the problem of sitting on land which is already mostly hard to defend geography-wise but is of enormous value, both practical (the Whites are going to need a breadbasket to allay the famines plaguing civil war-era Russia) and sentimental, to the White leadership while also being surrounded by enemies and not actually having the strength to defend their claims. That's a very bad spot for any would-be country to be in for sure.
As for Poland, I think the Western Entente and Russia will both insist on a Curzon Line border. The former twisted Kolchak's arm into acknowledging Polish independence IRL and I don't see any reason as to why they'd treat Nicholas III any differently, but they never said anything about where the border should be in that agreement to my knowledge. And while Poland might be lost to them in the short term, I can't really imagine die-hard Russian nationalists like Kolchak & Denikin not arguing for pushing the border as far west as they can & laying claim to Galicia-Volhynia as 'historical Ruthenian, and therefore Russian land'.
Demands like that might be easier to get through if Poland can get a more ethnonationalist mind like
Roman Dmowski in charge; he seems to have been more willing to accept a smaller but more ethnically homogeneous Poland compared to Pilsudski's dream of a 'Promethean' Intermarium to replace the PLC, not to mention he actually supported Russia against Germany previously. Might be for the best for Polish-Russian relations in the long run, Russia will already be seething over both the territorial losses in the West and the probable loss of much or all of their Pacific coast to Japan, if Poland insists on the Treaty of Riga borders from OTL that's going to 1000% guarantee a future equivalent to the Molotov-Ribbentrop pact between Anastasia's Russia and whoever takes power in Germany (as long as it's not the commies, but that doesn't seem all that likely with no USSR anyway) to kill Poland later.
The POD requires only Anastasia to survive, any of her older sisters also surviving would enormously reduce her importance as they'd come ahead of her in precedence. I think Maria (the third sister) was the one you're thinking of rather than Olga (the eldest), but she was probably the hemophilia-carrier out of the four which really doesn't bode well for her survival after being wounded, even if she isn't beaten to death by her executioners as happened historically. The second sister Tatiana (historically 100% killed by a bullet to the back of her head)
apparently had a teenage crush on a young cavalry officer named Dmitry Malama who then went on to die fighting the Reds as part of the Volunteer Army in the south, so besides Diterikhs himself there's a good candidate to avenge the Romanovs personally by going after Yurovsky ITL at least.
I'm inclined towards pessimism re: the ability of the 'Ultras' as you call them to seize a Yenisei border, tbh. None of them really strike me as great or even particularly good generals - Diterikhs to my knowledge certainly had great aggression and zeal, but not much in the way of tactical or strategic finesse, and Semyonov was basically a bandit chief whose army was the most exploitative & prone to marauding even in friendly areas out of all the forces gathered under Kolchak's banner historically (yet another reason why the latter hated him, he tanked civilian support for the Supreme Ruler everywhere he went). Ungern-Sternberg more or less had both their problems with extra insanity on top, and of course we know none of them lasted very long against the Reds after Kolchak's downfall IOTL. The greatest advantage enjoyed by the Pepelyayev brothers or whoever else is sent to fight them ITL may not even be the resources from European Russia being transported on increasingly long & painful trips past the Urals, but the promise of a relatively sane & predictable administration that might well seem downright benevolent compared to the 'eccentric' and/or overly pillage-happy trainwrecks these guys are likely to preside over.
So yeah, Diterikhs & Ungern-Sternberg being sent off to die gloriously against the other Whites seems reasonable enough. I guess Semyonov, as the most rational of the bunch and a historical longtime Japanese collaborator, would be Tokyo's preferred & most reliable candidate to actually rule the protectorate they'd impose on the Russian Far East (I'm guessing at a border around or east of Lake Baikal, which was where Semyonov had his fiefdom IOTL). Based on how he 'ruled' Transbaikalia historically I can't see him being a particularly efficient or corruption-free warlord, but it's pretty telling that a mafia state based out of Vladivostok might well unironically be the best the citizens of that region can hope for compared to how Semyonov's buddies would have ruled.
One other interesting development to consider in the east might be the flight of some Reds to China, which would make for a neat mirror to the surprisingly substantial White Russian exile community at Harbin (as well as White Russian troops fighting for Chinese warlords like Jin Shuren & Zhang Zongchang) IOTL.
The Soviets sent help to Sun Yat-sen historically and were originally close to the KMT (Chiang Kai-shek even trained in Moscow in 1923 and his eldest son married a Soviet woman), maybe Mikhail Borodin et al. could flee through anarchic and sparsely-populated Turkestan to join the KMT in this timeline instead. Of course Sun was never a commie himself, but maybe a greater Red diaspora in China could prop the left-wing of the KMT (Wang Jingwei, Sun's widow Soong Qingling, etc.) up enough to gain an advantage over the right-wing represented by Chiang and others, which would have big butterfly effects on not just China's future internal dynamics but potentially the broader geopolitical situation with Anastasia's Russia & Japan.