The defence of ignorance of the law has been used, not to disprove guilt, but in sentencing

An accused is sentenced after being found guilty, yes? Ergo, ignorance is not a defense but could be argued as a mitigating factor.

In any event, I feel that any educated adult (educated to at least secondary level) who works on a tourist visa in any country has only him / her self to blame for any consequences; regardless of whether they are familiar with the intricacies of the host country’s laws and regulations.

It’s undeniable, unarguable basic common sense that a tourist visa is for tourists, not workers.

first we have to consider if China operates by the rule of law or rule by law.

The expressions; 以法治国 "governing the nation in accordance with law" and 建设社会主义法制国家 "building a socialist rule of law state" were incorporated into China’s constitution in 1999.

It would still be helpful to see the statute that states what documentation is to be carried.

You’ll probably find some reference to the actual, legal requirement among the 15 pages of China’s laws and regulations on this website:-

http://www.chinalaw.gov.cn/article/english/Regulations/Laws/