I wonder if a better chance of this happening is a TL where:
Germany for whatever reason does not invade the Soviet Union (Hitler dies early or something) but tries a blockade England/ Mediterranean strategy which doesn't work. Japan comes in as OTL. USA comes in as OTL but focuses even more on Europe.
With Germany totally preoccupied with air and naval construction and suffering a gigantic African failure in May 1944 (like Tunisia May 1943 OTL but worse) and a May 15th 1944 coup against Mussolini. The USSR senses weakness and jumps in. War ends in May 1945 in Europe around same front lines as OTL.
However in the Pacific, USA is running about 3 months behind OTL due to focus on Europe.
USSR invades Japan early in this TL, even before Europe has ended in March 1945. Japan surrenders in July 1945 before bomb has dropped.
USSR doesn't have the history of Lend/Lease and conferences and the west has a grudge for the USSR treating with German/Japan for so long. So there is no spirit of cooperation.
As far as the local populations in these places and how mad they are. How much difference is there in being a SSR vs. an "independent communist state". I mean the Ukraine had a UN diplomat in OTL, then so could these. If you are in Poland would the flag flown be a Polish SSR flag or the Soviet flag? Perhaps the USSR could as suggested offer these SSRs a few more bits of independence.
There would be resistance but this USSR would be much stronger to handle it, at least initially.