They explain it as a circumstancial movement for creating a buffer zone against Germany as the Soviet Union had tried repeteadly to make an alliance against the nazis with the western powers which didn't accept, leaving the SU alone and isolated. They made the pact for gaining time because they knew the germans were going to attack them sooner or later as Hitler had spoken several times about Lebensraum and the right of the germans to take the soviet territory for themselves exterminating the slavs.

We are wondering whether the part about USSR invading Poland and other states in 1939 is even in Russian textbooks or does WW2 in the minds of Russians start in '41 with the Nazi invasion of the USSR?

Not really an answer but the book "The Sword and the Shield" by Christopher Andrews and Vasili Mitrokhin provides some insight into Stalin's paranoia of an Anglo-Nazi alliance and how it shaped policies and directives of Soviet intelligence prior to the war.