1) There was nothing "semi-binding" about it. Hitler and Stalin (actually, their foreign ministers Ribbentrop and Molotov) had signed a 99-year non-aggression pact, promising that neither one would attack the other. Its language was quite unambiguous.

2) No factors changed. The whole thing was a ruse, and Stalin should have seen through it. Hitler spent most of the 1930s talking about how Germany needed "lebensraum" (living space) and how they would get it only in "the east"...meaning, Russia. He did the non-aggression pact hoping that it would cause the Soviet Union to take a less defensive posture. In the end, of course, it didn't work...but not before 25 million Russian citizens were either butchered by the Einsatzgruppen or deported to extermination camps because they were "sub-humans" (read: of the wrong "blood") or locked away in prison camps where only a very small percentage survived. Whatever you can say about Hitler, you can never legitimately claim that he didn't make his intentions quite clear very early on. Mein Kampf and his speeches spelled out exactly what his intentions were.