The point I've been trying to make, perhaps badly, was that China "punched way below it's weight" with regards to the war of 1937-1945. The army that invaded Korea in 1950 was not only willing, but eager, to offer battle. The change in offensive spirit cannot simply be chalked up to mere 'revolutionary fervor'. The Nationalists and Communists during the Second World War avoided battle, husbanding resources for a war against each other, despite the fact that Japan threatened China with extinction.
As for the Russians 'getting lucky', that's a fallacy. The Nazis did, of course, ignore the rasputitsa (as the Russians call the seasonal floods), but in the end, what saved Russia was the fact that germany simply ran out of soldiers. Germany could not continue to advance into the Soviet Union and still maintain enough cohesion and concentration of force to hold onto thier gains. Allied aid, especially American, kept the Russiams on their feet until they were ready to take the fight to scattered nazi armies at the end of an extremely long supply line.
The Japanese found themselves in a similar position. They simply could never muster enough troops to both occupy China in any meaningful way and so the war there became little more than the parading of armed men. The Chinese, if suitably motivated, could have pushed the Japanese out of China at any given moment, I believe, Chinese numbers and Allied supply making up for Japanese "technical superiority" (remember, this was basically the same Japanese army that had lost to the Russians in 1939, who were not all that technically advanced except in armor. Russian numbers and concentration overcame Japanese skill and technology).
You say the allies would not have won without the Chinese, but I say it's more like they won DESPITE the Chinese.
At the end of the day, the Russians, French, Poles, etc, fought, and usually well, unlike the Chinese.
I'd agree that both Chiang and Mao underestimated the Japanese, but i'd disagree that both didn't fight well. You also OVERESTIMATED the support China got from its allies.
The fact is, if there is no Russian front, and if there is no China containing the Japanese, the Germans would have only had to deal with one side, and the Japanese would concentrate its energy on naval battles against the US, etc. instead of having to spent resources trying to control China (which it couldn't). I'd say that 20 million estimated casualties from China means it did resist and resisted pretty hard. (that's 20 million including civilian casualties) In fact, if the bulk of its army isn't tied up in China but rather in Japan, I don't think 2 atom bombs will cause them to surrender and US will have to face a huge casualty bill in order to win (if it would win against Japan, had Japan not gotten itself in China).
But of course, strategically, the Japanese can't control the world without Asia, and the key to Asia dominancy is control of China.