The problem with that is both Poland and the British and French were becoming more hostile to Germany and less likely to practice appeasement, especially once Chamberlain and his supporters are gone.
The end of appeasement was due to a combination of factors, including Germany's aggressive foreign policy and rearmament, right wing government, poor international standing, the annexation of Czechoslovakia, and a public much more supportive of war than it was in the early-mid 30s. Only one of those factors won't still be around.
As I mentioned above, while Czechoslovakia was an important turning point, even before that the public was increasingly hostile towards Germany and was only going to get more radical. By 1942 Britain and France would be capable of war and would have publics and cabinets willing to declare it in support of Poland. This would be less out of love for Poland and more to stop an expansionist and blatantly aggressive Germany, which would exist with or without Hitler and would be disliked by Britain and France.
Not really. I have to absolutely disagree that the end of appeasement was a foregone outcome by 1939, no matter the circumstances. The disillusionement of Entente ruling circles and public opinion with appeasement was not a process happening on its own for no good reason but in large part the reluctant realization that Hitler's aggressive plans went much beyond the revision of Versailles' settlement and satisfaction of Germany's long-standing irredentist claims (which the Western powers had generally come to regard as acceptable by the 1930s).
Since a non-Nazi Germany shall have no such grandiose imperial plans, and its irredentist goals shall match its stated intentions, it may push its claims on the Sudetenland and the Corridor in a rather more diplomatically moderate way than Hitler (to make a typical example, they may push for an internationally-monitored plebiscite in the Sudetenland and Danzig, which they would win by a landslide, instead of immediate military occupation like Hitler did). Moreover, the slower pace of German rearmament shall make the Western powers less alarmed (and make them rearm themselves at a slower pace too). Last but not least, Germany shall leave rump Czechoslovakia alone, showing itself to be trustworthy.
The combination of those factors shall in all likelihood make it so that appeasement remains the preferred foreign policy strategy and Germany's international standing shall be substantially better. The Entente public opinion shall remain mindful of the huge blood price that it paid to fight the Great War and unwilling to go that way again if it can be avoided at all, even if Britain and France get better prepared militarily. With a more moderate Germany, there is no good reason why the Western public should get more warlike. Ruling circles shall remain mindful of the bigger strategic picture, with the power of the USSR on the rise thanks to its industrialization and rearmament and eager to use a satisfied Germany as an anti-Soviet bastion. 1939 Germany plus the Corridor is in no way a threat for the European order at large, as long its foreign policy remains responsible.
In these conditions, Chamberlain's faction shall remain in control of the British foreign policy throughout the early 40s, no way Churchill and his ilk shall gain control of the UK government in lack of a general war. Likewise, there is no good reason why anti-German hawks should gain control of the French government. A non-Nazi right-wing regime shall be politically acceptable to Western public opinion. In the late 1930s, right-wing authoritarianism was pretty much the norm for most of the continent.
I disagree with your third paragraph because Nazi Germany had an extreme effect on Soviet foreign policy; prior to its genesis Soviet policy was almost totally isolationist beyond military cooperation with Weimar Germany, which was quickly becoming unnecessary, and trade with various nations. Nazi Germany forced it to seek allies in the face of an aggressive Germany which sought its destruction.
Prior to the late 1930s, the USSR was isolationist essentially because Stalin was well aware that Soviet Russia was woefully weak and not prepared at all to fight a major war with the other great powers to expand the Soviet sphere of influence. He was not any pacifist, just more opportunist and cautious than Hitler. By the late 1930's and early 40s, Soviet industrialization and rearmament was changing the picture and Soviet foreign policy was getting more aggressive because its leaders knew they could afford to be more forceful. E.g. see the Soviet-Japanese Border Wars, the Soviets were willing to fight them certainly not because of the existence of Nazi Germany.
it's very much possible that the ruling generals in Germany won't be as inclined to carry out such a pragmatic betrayal of their values, or at least unanimously agree to it.
True, nothing is completely certain. However, the strategic benefits of a temporary German-Russian alliance of opportunity to take down Versailles Poland remain evident regardless of whom is charge in Berlin and Moscow, as long as the ultranationalist Polish leadership remains hostile to both great powers. Those generals have managed covert military cooperation with Soviet Russia for a decade, so the idea of a limited pact with the Bolshevik devil is certainly not outside their mental horizon.
Even at the start of Barbarossa Stalin considered it likely that the attack may have been launched without Hitler's consent by his generals and that it still could be called off.
More than anything else, Stalin was unwilling to believe that Hitler would take the risk of attacking the USSR before defeating Britain. He was still expecting that Hitler would betray him in the end (and for that matter, Stalin was planning to backstab Hitler in the end himself), just he could not believe him so foolhardy. Anyway, this has little bearing on whether a non-Nazi Germany and the USSR may be willing to set up a partition deal of Poland. Such a pact causes little risk to both nations and benefits them greatly by ensuring the satisfaction of substantial irredentist claims.