But Soviet Intelligence officer Richard Sorge was able to inform Stalin that Japan had no intention of invading the Soviet Far East, unless Moscow was captured by Germany and they had a 3-1 advantage over Soviet forces in the region. And as Japan re-assigned forces from Manchuria to China and then to South East Asia, such an advantage was never going to come. Having reported very closely the planned date of Barbarossa (though dismissed at the time by Stalin) his reports that Japan would not invade were believed and allowed a good estimate of what forces could be transferred to the Battle of Moscow, without leading to an invasion opening a second front.
Plus it was winter. I don't believe that the Kwantung Army would have started an offensive into Siberia at that time of year (Nov/Dec 1941), if they had judged that the Soviet forces facing them were weak. And Japan was by then committed to invade South East Asia and go to war with US and UK. Once Pearl Harbour happened then Stalin knew with some certainty there would be no stab in the back from Japan, and that the amount of support from US would increase.
It's interesting to note that the Soviet Japan Non-Aggression Pact was signed publicly a few months before Barbarossa. The Japanese Foreign Minister visited Moscow openly, and the Pact was signed in front of the diplomatic corps. This was not some secret deal like the Molotov Ribbentrop Pact. Yet Hitler still went ahead. The leadership believed that they only had to "kick in the door and the whole rotten structure" would collapse. Hopelessly miscalculating the military capability of the Soviets and the willingness of the ordinary citizen to sacrifice themselves in a way they never experienced in Poland or France, and the seemingly unlimited resources of men and material (backed by US lend lease). The loss of the whole Sixth Army at Stalingrad was something which Germany could never afford, and most military people believed that defeat meant they would lose the war. The Soviets could, and did, lose many army sized forces and still fight on, and in December 1941 launch a counter offensive. And throw away huge numbers of troops a year later in defending Stalingrad while simultaneously building up the forces needed to encircle the Germans.
The fact is that no amount of theorising about what Germany could have done to gain victory is needed. The decision was made by a madman who believed it was his historical destiny to destroy Bolshevism, and that as a so-called "inferior race" Slavs were not capable of building tanks and planes that could match their own, never mind have the engineering resources to build them in huge numbers. At no point did any German planning take any account of the industrial capacity of the Soviets, and they clearly didn't believe it mattered one way or the other whether Japan invaded, nor whether Japan's actions brought US into the war.
