Keep in mind the topic is "how is China able to provide enough food for 1.4 billion people", which implies they must have some efficient or cheap way. And I'd agree "rice" probably is one of the reason.
However, you firstly used "Shanghai's white-collar" as an example, which is the least representative group for this topic (because how cheap the food is is less of a concern for the high-income groups).
You also mentioned obesity, which only dramatically increases when China got richer. Again, totally irrelevant to the topic. If anything, Chinese people probably eat less rice now due to the increase of variety.
I do not thing that being a dietician gives you much authority to comment on the cross-cultural socioeconomics of nutrition, sorry.
The claim that the Chinese are fat and unhealthy because they are force-fed empty calories remains unproven.
I don't question any of your data or even your conclusion, but do you really think WHITE-COLLAR workers having diabetes is a proof of " food is low quality empty calories"?
To me, it's more about the fact that lots of young people (or people in general) in urban area have poor lifestyle. Barely has anything to do with the lack of food quality or nutrition.
And I highly doubt this phenomenon is unique to China.