I agree with this idea; the view that invasion of Poland was start of this global war is rather Euro-centric.
However, as you said it is fuzzy: the hostilities were still somewhat limited even after the invasion in Manchuria; but total war broke out in July 1937 after the Marco Polo Bridge incident.
Invasion of Manchuria was 1931; 1937 was invasion of mainland China until Zhejiang and Hubei. In consequences, there was the alliance of Germany and China fighting against the Soviet attack on Xinjiang, for instance.
It directly contributed to sanctions such as American embargo of Japanese imports, and things like Export Control Act of 1940 and then the Tripartite Pact.
(BTW, thanks for asking; this made me read more about history in the area - I had no idea, for instance, that Soviet artillery and airplanes had attacked Chinese Muslim nationalist (KMT) troops with mustard gas in 1934 in the Soviet invasion of Xinjiang, nor that there were still White Russian troops that operated on same side with NKVD and GPU troops. What a murky business.)
Embargos aren't war, otherwise the West is at war with Russia right now due to it's annexation of Crimea and activities in the Ukraine. And does it really count as Germany at war with the Soviets if there were no German troops involved?
Similarly, if you're counting political events that foment war as being part of that war, then WWII started at least as far back as 1919, when Ferdinand Foch stated that the WWI peace treaty "wasn't a peace, but an armstice for 20 years".
The thing is that Japan's military adventurism in China was a regional affair, conducted between neighbours, whereas Germany attacking Poland brought in countries from all inhabited continents except South America.
For me, Japan's large-scale military attack in China in 1937 was more than a "political event".
It's not so clear-cut where policy ends and war starts, and when a war turns into a global war, but I cannot say that September 1, 1939, would clearly be where we draw the line. The fighting in China in 1937 (Battle of Shanghai) was actually bigger in scope than the invasion of Poland in September 1939, and with more casualties. Then the Winter War in Finland exceeded these, with big losses for USSR, and then large-scale war started on the Western front in the Battle of France in 1940 which was even bigger than Winter War in terms of casualties.
That muslims were members of the KMT and served in the KMT army means nothing. The KMT was not a muslim organization. It had no religious affiliations at all. It was a chinese nationalist political party.
I don't think I said KMT was a muslim organisation. It was a nationalist organisation, or a warlord organization - the allegiance of generals to the party was often quite nominal.
However, it bears some relevance to the matter that KMT-affiliated troops that fought against Soviet invasion in a predominantly Muslim-populated area were Muslims. I don't see here any particular parallels towards Islamic or Islamist movements today.
However, as you said it is fuzzy: the hostilities were still somewhat limited even after the invasion in Manchuria; but total war broke out in July 1937 after the Marco Polo Bridge incident.