I think, that if Poland didn't attack Czechoslovakia, Stalin will leave it alone (for a while). Also, Japanese beating Soviets is little bit far stretched.I think that Turteldove presents an accurate picture of the war, that is until the big switch. I think the fighting on the German Czech and Soviet Polish borders allows Britian and France time to prepare. They stop the German advance in France and Norway, after the fall of the Benelux and Denmark.
I think a campaign of unrestricted submarine warfare brings the US in or maybe Hitler declares war after Japan attacks the US as he did OTL.