You overestimate the corridor. First of all, Lodz, Poland's second biggest city (suffering heavily at the time; it was the textile industry centre pre-WWI for the Russian Empire, and was cut off from its markets, but still important) was definitely NOT part of the corridor, it was a part of 'core' Poland. There was no coal in the corridor, not much industry. It's importance comes from the sea coast.
Now, Poland having a deal with Hitler in aug is ASB, because: Hitler wanted the war and would have attacked anyways, France was no help with it's defensive strategy, and Polish government was stupid and blind. However, if you remove Rydz-Smigly from power and have someone like Pilsudski there, not '34 Pilsudski, destroyed by cancer but someone like Pilsudski from 1920, he could have agreed to give danzig and allow to build the highway (infact, it was polish idea). Danzig was of no worth to Poland really, only symbolic matters... Hitler, of all german governments between the wars, was the one Poland could have a deal with easiest. Hell, giving him the corridor and Posen plus satellitization would be better than what happened OTL.
Secondly, Stalin would NEVER strike without a deal with Hitler.