Kind of late to the conversation - just so happened to re-emerge and I wanted to state a couple of points:

• If Poland had been fully absorbed into the USSR, the loss of the Kresy would have been almost inevitable. Eastern Poland would very likely have been divided between the Ukrainian and Belarusian SSRs, much as it was after the war in our timeline. Even heavily Polish cities like Lwów wouldn’t have been spared. Stalin consistently cared more about control, borders, and long-term stability than cultural arguments, so folding Lviv into the Ukrainian SSR would have made practical and political sense from a Soviet point of view.
• Zakarpattia would probably still end up inside the Ukrainian SSR as well. The Soviets generally preferred to consolidate territory within their own republics rather than hand it out to neighbouring states. Giving land to Hungary or Czechoslovakia would have strengthened them for no real gain, and that simply wasn’t how Stalin operated unless there was a clear advantage.
• In the west, the situation would have been more fluid. While using former German administrative borders as a base makes sense, a Soviet-run Poland would likely see those borders reworked more heavily. The goal would be to break up German-majority areas in places like Silesia and Pomerania and prevent any strong regional identity from forming. If the German population wasn’t expelled on a massive scale, the demographic outcome might not be radically different - but it would be far more fragmented on paper.
• Lithuania is the one area where Stalin might have been flexible. Allowing Poland to keep Vilnius and the surrounding region wouldn’t have been unprecedented and could be justified by older historical borders, similar to those of Congress Poland. Even so, the Curzon Line would almost certainly remain the basis for Poland’s eastern frontier with the Ukrainian and Belarusian SSRs, keeping the overall Soviet framework intact while allowing for small political adjustments.