"vast majority" seems to be a bit of a stretch. The Kantogun Army guarding against the Soviet threat alone was about equal in size to the Chinese Expeditionary Army. Both of them were frequently stripped of men and supplies in order to reinforce other more active theaters. They got away with it because Chiang sat on his ass during most of the war. Yes the, Chinese theater tied down lots of troops and material who had to guard and occupy a huge land area, true. Did they tie up the "vast majority" of Japan's total military resources? I don't know about that.
Someone here posted details on Japanese troop deployments; if I find the link again I'll post it here.
As for the charge against Chiang Kai-shek; it's partly true but you need to keep in mind that the NRA was never in a position to launch a major offensive against the Japanese until 1945. It had enough resources to wage a war of attrition against the Japanese but not enough to actually inflict them a severe defeat (Chiang Kai-shek did sacrificed the best of the NRA in Shanghai back in 1937, but it's doubtful that even if these forces had fared better that the NRA would be victorious in a major offensive against the far more well-armed and well-organised IJA). And as others have already pointed out, Chiang was anxious to conserve his strength for the showdown against Mao Zedong. This contrasts with the US war effort; the Americans had far, far, far more industrial and technological might at their disposal and had no internal enemies.