I think that if "Operation Unthinkable" as Churchill planned came to existence better question would be how badly WAllies are beaten...
I doubt Poland will get ost preussen, this plan centred on rearming the Germans and using them heavily, assuming it's successful I doubt the Wallies are going to tell the Germans who just retook Konigsberg to now give it to Poland.The two aren't mutually exclusive. For instance, here was FDR's proposed postwar map of Germany:
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My understanding is that the Western Allies wanted to divide Germany into several states after the war, but weren't as enthusiastic about moving Poland's borders so far west. However, with the Soviets occupying East Germany and Poland, there was little they could do to stop them. However, if the Soviets are pushed out of Central and Eastern Europe immediately after WWII, we could end up seeing something like what is depicted above. However, no matter what happens, I feel like Poland will probably end up annexing East Prussia - ALL of East Prussia, with obviously no Kaliningrad Oblast.
Though this raises the questions of Poland's postwar borders in the east. What more could they possibly gain, beyond restoring their pre-1939 borders there?