On the issue of the Chinese using German helmets: yes, they did, but they also made use of quite a few other types of headgear, including the equally iconic British-style helmet and the French-style helmet. Keep in mind:
1. The Chinese KMT (“Nationalist”) party had its own flirtation with fascism (and a “blue-shirt” paramilitary arm at one point: fascism was kind of the “cool new thing” in many parts of the world back then) and yes, did have German advisors for some time.
2. The Chinese desperately scrounged for any useable equipment they could find and thus I wouldn’t be surprised at seeing weapons/ equipment from pretty much anywhere in WW2 Chinese hands.
3. The German helmet was one of the original “Big 3” helmet designs coming out of WW1, along the the British Brodie and French Adrian designs. At one point, just about every army in the world had adopted one of these three:
French Adrian: Russia (& the USSR), Italy, Greece, Belgium, Luxembourg, Poland, Romania, Serbia (&Yugoslavia), Brazil, Mexico, Siam, and Japan
British Brodie: US & the British Commonwealth countries (obviously)
German Stallhelm: Austria, Ottoman Empire, Spain, Hungary, Argentina, and Chile