I'll ignore your assumption that a defeated country whose capital is taken over by the enemy ceases to exist and just state the facts.
You'd better read what I said instead of deducing my assumptions.
What I said is that the state seizes to exist if there is no government
at all! The government of Poland didn't go out of the capital to the eastern part of the country. They were evacuated to London. You can't expect a foreign neighboring country to remain neutral.
Of course the Red army will move in to create a buffer zone if the Polish government deserted from their duties.When Hitler occupied Czechoslovakia in 1938 some regions of this country
in the same time were occupied by Polish troops. Poland invaded and annexed Zaolzie territory part of Southern Slovakia. Why would you expect one year later in a similar situation the Soviets to act differently?
They were so eager to create the buffer zone that they attacked Polish forces stationed along the border and proceeded to drink Vodka with the Nazi, coming from the West. The prisoners were put in concentration camps and 6 months later, in March, murdered and buried in mass graves.
Poles and Czechoslovaks were conflicted about Zaolzie 20 years before that, simply because it was inhabited by both nations and both thought of it as their land. The Allies proposed a plebiscite to be held in 1919, to allow people to decide which country should claim the land, but finally it didn't happen and the land was divided in Spa (1920). As a result over 100 thousand Poles were forced to leave and of course most of them wanted to go back.
Zaolzie was taken without a fight 18 years later, with consent of the foreign minister of Czechoslovakia, so it's hard to compare this to what the Soviets did in 39.