My last resort was to call the airport in Vienna again, since there was the fourth flight on this new route today. They still werenīt allowed to connect me to the check-in manager of CZ, but the very nice lady agreed to call the CZ check-in on the internal line and ask on my behalf. The answer was the same as from the immigration officer in Guangzhou: No Visa needed, if the whole itinerary in and out of China is with CZ and the total time in China is below 24h. He just advised that I would probably need a Visa for Vietnam (which is half true, German passport holders get 15 days VOA in Vietnam). Nevertheless, they donīt deny boarding to passengers without visa and it should work out fine. Still not hard facts, but pretty much the best I can get for now.