This was part of the secret protocols of the MR pact. Europe was effectively divided between the two powers, with the Germans declaring complete “political disinterest” with the regions of Eastern Poland, the Baltics and Besserabia. Look familiar? That’s also a list of the places the Soviets invaded after the MR pact was put into place. Finland is never explicitly mentioned AFAIK, but I presume it was simply a case of it being so obviously in the Soviet sphere there was no point making it clear.
Either way, the MR pact was the lynchpin that allowed the Soviets to freely invade their neighbours without immediate risk of Germany jumping them while they were busy.
This is very questionable. Hitler made his views on an independent Polish state very clear in 1930, stating that they (And the Czechs) were a “rabble not worth a penny more than the inhabitants of Sudan or India. How can they demand the rights of independent states?”
Even if we don’t take him at his word, you need only look over at the Sudetenland to see what happens when you give Nazi German the small territorial concession they want.
And while Danzig itself might be majority German and in favor of annexation, the overall Polish corridor was not.
And finally, as already mentioned above, the MR pact explicitly divided Poland between Germany and the Soviets. I doubt they’d simply blow off the Soviets so directly even if they did get Danzig.
Wars often have more than once cause. Yes, Germany Barbarossa’d for resources needed to fight the allies, but it wasn’t the only reason, or even arguably the main one. The MR pact (yet again) provided Germany with substantial raw materials from the Soviets in the form of trade deals. And this did include oil. Huge amounts of it. Attacking the Soviets hurt their oil income substantially, and that wouldn’t have evened out until they hit the Caucases.
No, the primary factor for attacking the Soviets was ideological. Hitler hated communists, and slavs, and most of all communist slavs. He also wanted to expand Germany, literally to make it bigger, and where better to expand than the vast, resource rich territories of Russia, whilst also eliminating one of his most hated rivals.
What does that have to do with anything? Stalin mobilized his troops because his closest territorial rival had just finished eating two European states and was eyeing up a third right on his border. The rather obvious conclusion being that the second Poland falls the Soviets would be squarely in the Nazi’s sights. Moving troops around serves two purposes, readiness in case the worst happens and deterrence to prevent said worst.
And the horrific losses the Soviets suffered during early Barbarossa had many causes, but I assure you a lack of bunkers was not one of them. Horrible doctrine, being filtered down through inexperienced officers, to underequipped troops, relying on horrific logistics that basically collapsed the second any strain was put on it, under complete German air dominance, accounts for most of it.