But short of that I think they could have won if Japan even made the gesture of attacking Russia.
The forces in the ‘far east’ didn’t cause Hitler to lose, Hitlers decision to attack Stalingrad and how he did it destroyed his chances in 1942.
In 1941 The winter caused Hitler to lose because he didn’t prepare for it and underestimated the Soviet army. Those forces were just icing on the cake used for the foiled counteroffensive, AFTER the Germans failed to take Moscow. The battles just before Moscow at Vyzma are the ones that cost the Germans too much time, so they didn’t get to Moscow in force after fighting 5 Soviet Armies.
“Contrary to German expectations, the encircled Soviet forces did not surrender easily. Instead, the fighting was fierce and desperate, and the Wehrmacht had to employ 28 divisions to eliminate the surrounded Soviet armies, using forces that were needed to support the offensive towards Moscow. The remnants of the Soviet Western and Reserve Fronts were able to retreat and consolidate their lines around Mozhaisk.[22] Moreover, the surrounded Soviet forces were not completely destroyed, as some of the encircled troops escaped in groups ranging in size from platoons to full rifle divisions.[23] Soviet resistance near Vyazma also provided time for the Soviet high command to quickly bring some reinforcements to the four armies defending the Moscow direction (namely, the 5th, 16th, 43rd and 49th), and to transport three rifle and two tank divisions from the Far East.”
I hardly think “three rifle and two tank divisions from the Far East” were the cause of the German failure. The weather and Hitlers unprepared logistics past a campaign of over 4 month duration were catastrophic.
If Japan attacked Russia it would have done nothing except expose yet another front to them and cause a 3 front war for Japan. The balance of the Soviet forces on the border were more than enough to wipe out the japanese Kwangtung army as they did in 1939.
Japan would gain nothing but another defeat, and tie up units that were used after Dec 41 in a protracted battle. Declaring war and doing nothing was not an option either. Stalin would go on the defensive if he was engaged with Germany. If Japan didn’t get her feet wet, the ploy would fail and 5 divisions still head toward Moscow. If they committed, it would have hastened japanese defeat in Asia. Stalin had more than enough forces for both once Moscow was protected.