Only problem I see with the scenario is here. Why would Hitler reject this Soviet warning? Poland is essentially defeated and by conceding the eastern half to the Soviets he accomplishes his goal of preventing a two front war. The French Saar Offensive is going nowhere, they just didn't have the material necessary to sustain an offensive effort yet. Once that stops, I would think the Soviets would immediately want a way out and cut a deal as in OTL.
Despite this, it's still interesting to explore a two front war in 1939. Very nice analysis in your OP.
Honestly I just threw that in there for dramatic character. With Germany having already rejected the British ultimatum to withdraw from Poland, the French launching an offensive in the Saar, and the Anglo-French agreeing to much of the substance of what the USSR wanted (Military/political alliance, "indirect aggression" against the Baltics, etc.) I doubt Molotov would still be trying to cut a deal. The USSR's preferred option was an Entente 2.0 as long as the Anglo-French countenanced its imperialism in Eastern Europe.
As for why Hitler would reject an ultimatum if it was issued, here's my thoughts:
1. Hitler was in a manic phase after the swift destruction of the Polish armed forces. He immediately began plotting an attack on France in November despite vast material shortages, and ranted to his generals about how Germany had "three times the resources" of WW1 with which to launch an offensive. He wasn't rationally considering the balance of power at this point.
2. Hitler, as he noted repeatedly to his generals in 1939, was unhappy that he had to turn East before he had dealt with the West but was willing to see it through. That's why he offered a "peace deal" to the UK/France on October 6, 1939. He was almost certainly plotting to convince them to give him a free hand to attack the USSR/Baltic States next. A war with the USSR strengthens his hand, in his mind, by allowing him to present himself as the "defender of Europe" and get a peace deal with the UK/France.
Hitler was a constant gambler and made decisions impulsively and emotionally based on the "truthiness" of what seemed right, not facts. Sometimes he settled down and made an informed decision, but when he was in a manic phase he was fixated on his own invincibility and driven by preexisting fixations/biases.