that just clarified that negotiations were effectively over.
Nah, that was on October 9th when the Japanese did indeed decide that negotiations were over.
Japan's planning for war came specifically after the occupation of French Indochina in September 1940 triggered US sanctions.
Japan's planning for war against the US predates it's war in China, much less the occupation of French Indochina.
Both of those things do not happen per OP, so Japan continues on in China without that issue driving planning and holding back the IJA in Manchuria.
Instead, what is holding back the IJA in Manchuria is a furious debate over whether to take advantage of French collapse, go north, or just continue doing what they've been doing. Either the last wins out or the debate lasts so long that it winds up winning by default
BobtheBarbarian disagrees that that would have been an issue
Bob has such a positive interpretation of the IJA and their plans that I'm not sure he's actually read Nomonhan like he says he has, because Coox is pretty consistently denigrating of their capabilities, what they learned, and the prospects of both the Go North plans political and military chances. For example, he claims that the IJA had learned their lessons about mechanization and firepower at Nomonhan, but Coox says otherwise saying that "Two years after the experience at Nomonhan, IJA ground divisions were still markedly inferior to Red Army divisions with respect to firepower, mobility, armor, and air support." (Page 1053)
I should also note that, if your going to invoke him, Bob agrees with me that a Japanese attack would hardly doom the Soviets against the Germans and would be anything but a "Axis victory" scenario.
There were a lot of divided attitudes
Within the Go North party, yes, as even their people were pointing out that the withdrawal of Soviet forces were "far fewer then the optimum the Japanese army had hoped for." (Coox, page 1045) That weakened their defence against the parties who were against the Go North option... who were well and united
opinions would be different without the embargo weighing on Japanese leadership.
Oh, the embargo would weigh on the Japanese leadership because the Japanese would be aware that it would be a natural US response to their actions to impose such an embargo.
I'm sure there will be contingency plans, but if they are planning war against the USSR they'd wrap that up first and expect Germany to help them in case of an Allied attack.
Japan would be aware by September that the war with the USSR would be a long one, which they would have no prospect of wrapping up quickly (even they later concluded that their ability to project force beyond the Soviets immediate border defenses was hopelessely optimistic), and they would be exposed to a Allied attack in the meantime. They would also be aware that Germany would be physically incapable of helping them.