Conflict with Poland is inevitable, whether or not it's a focus is more of a tactical question than anything else, in hindsight it's quite obvious that the Soviets were by far the bigger danger.
Anyways, if Melnyk prevails in the internal struggle (and I don't really have a POD for this unfortunately although Konovalets' fate might be promising) you might see Stsibors'kyi's ideological ideas permeate Ukrainian nationalism far more than Dontsov's, which would be great, Stsibors'kyi was not anti-Semitic, and was ambivalent towards Germany. Dontsov was also a Polish asset, and after that a German one. Dontsov's value, intellectually, spiritually, politically, etc. is low, Stsibors'kyi's is better morally and practically.
The real question is which approach is taken, the 1943 playbook, i.e. resistance against Germany, the USSR, and the AK whenever they bump into each other (until at least the war is over because pre-'45 cooperation is unlikely without massive prewar PODs) or the WWI playbook, i.e. form a new version of the Sich Riflemen aka the 14th Waffen SS Grenadier division/Ukrainian National Army, and hope the Germans and Russians both collapse leaving the UNA and OUN to pick up the pieces.
Personally, if you want the moderate faction of the OUN to hold together Stsibors'kyi, Kapustians'kyi, Iarii, or one of the other older generation of exiled leaders are better choices, the former two are the better picks imo, Iarii went with Bandera, and was capable of appeasing both factions, but was also of non-Ukrainian ancestry so might be a controversial pick. All three names I mentioned were, iirc, also married to Jewish women apparently, which would be interesting vis a vis Germany. Stsibors'kyi might be, also, too much a theoretician for a leadership role, although that's not my personal appraisal. Kapustians'kyi I know not as much about, but could be a sensible pick, a former general of the UNR.
It's hard to say, Melnyk was an Abwehr agent but was also repressed by the Gestapo, I think it's plausible that post-Barbarossa, he tries to form an independent government backed by the OUN, church, and prewar political establishment, if this is suppressed he has the choice of accepting it, fighting immediately, or most pragmatically just doing the OUN-B IOTL play, go underground, feign collaboration and then when the time is right, resist. This gives the OUN time to build up its network outside western Ukraine, Stsibors'kyi was infamously assassinated by OUN-B operatives on this exact mission. Without this conflict the OUN may become far more entrenched outside of the West, which could be very interesting.
Melnyk did try to form a rival government to Bandera's, but I don't know as much about that attempt, he seems to have been more cautious politically. So the declaration of independence might not have happened, or been delayed somewhat, who knows.
Let's say Melnyk tries to form a government, is suppressed, and decides to do the IOTL OUN-B play. When his version of the UPA is formed, probably again at some point after Stalingrad, they've also infiltrated central/eastern/southern Ukraine much more effectively than IOTL. Without infighting they're more able to resist the Germans and achieve some more success in liberating territory, Melnyk probably doesn't do Volyn but of course there'll still likely be Polish-Ukrainian conflict, which will be quite bloody, if less organized.
Now, once the Germans retreat from most of the UkSSR the OUN forces there probably don't fare well, but having more effectively been entrenched and without bloody fratricidal conflict they make more of a stand. By the time the war is definitively over, perhaps there is no Ukrainian National Army/14th Waffen SS, or it isn't nearly as significant, but the OUN-UPA is much stronger in western Ukraine. The swamps of Volyn and the Carpathian mountains are great insurgency country, and they were Iotl able to bloody the soviets and maintain violent resistance into the mid 50s even if by 1949 defeat was evidently inevitable (Iotl they were basically hoping for an operation unthinkable scenario). ITTL organized resistance is more effective, even if it isn't enough to win, the extra toll it takes on the Soviets. This stronger showing not only in Western Ukraine, but in other parts of the country, could have interesting ramifications come the collapse of the Soviet Union. Ukrainian nationalism might be slightly stronger not only in Western Ukraine, but also in other parts of the country. Even if, post 1951 the Poles and Ukrainians do a ceasefire or alliance, they're not "winning." In the conventional sense, but keeping anti-Soviet politics relevant for as long as possible is a win for sure.
Post '91, if the poster children of Ukrainian nationalism are Konovalets', Stsibors'kyi, and Melnyk, with the latter two replacing Bandera and Dontsov, that's certainly more appealing and less controversial. Relations with Poland will be a bit better probably, but Volyn has never seriously threatened Polish-Ukrainian relations iotl.
My feeling is that probably we're not changing the results of Soviet history too much, and post '91, idk if any Ukrainian elections are necessarily going to be far different, although I'm not too sure about that. Certainly, the longer Ukraine is independent the more that changes, as per IOTL, but this less controversial OUN history will be more appealing. Of course, modern Ukraine will necessarily gravitate towards democratic and pro-western politics, and while it may be (slightly) accelerated in this timeline a revived OUN is unlikely to win an election.
Now, could this have butterflies that prevent say, Yanukovych? Maybe, idk.
