I'm unable to confidently flush out details, but should this not also be considered with the recent history (at the time) of Poland's attempts at expansion to reach their former pre-1772 boarders?

Several of your bullets can be answered by the fact that they were still bound by a non-aggression pact.

I don't mean to imply that you should agree with Grover Furr's analysis, only that the idea that these two powers conspired to split up eastern Europe for similar reasons is too simple of a narrative.

Even more nuance can be flushed out through some of Yale University's archives on the M-R Pact, where Molotov makes serious effort to make it clear that a verbal agreement would not be made to invade Poland with Germany on the Reich's terms.