#1 Austria wasn't "taken" they choose to join Germany
#2 Czechoslovakia and Germany always had a small problem with what belongs to whom, if Stalin was suprised that Germany finaly settled the dispute I would be impressed with how stupid the communistic party was.
#3 Nothing brought the USSR to the table, they were already sitting there, since after WW I, right next to Germany. And I don't think the USSR seen an Invasion of Poland as a big risk to the Soviet Union, as Poland is like Czechoslovakia clearly in the "german sphere" to use your wording, and not in the russian.
You sound like you think the "final" border of the USSR existed pre 1945, News: They didn't, the Russian empire stoped right at the Ukraines border.