The Soviets invaded Poland to weaks late according to the treaty with Germany, because they were held back in Belarus, the Baltics and Ukraine

You’re misunderstanding the post. Those territories at the time belonged to Poland. It is not until two weeks after the Nazis invaded, at a point when the Polish government collapsed, that the Soviets entered those “Polish” territories that now we consider Ukrainian, Belarusian and Lithuanian, and a very minor part of what nowadays we consider Poland.

Again, what was the alternative to Soviet presence in said territories after Polish government collapse?