It was decisive; if Japan had attacked in the east while the rest of the Axis did so in the West there was no chance of saving the USSR. By the end of the war much of the Soviet army was composed of non-European troops.
What makes you think so? The history of East Front in the West is written by the losers who wanted to look good.
It is bad winter, human waves and crazy Hitler by their book.
The fact that they weren’t up to tast isn’t acknowledged.
The Japanese did better analisys than the Germans and didn’t attack for that reason.