I'm a native Mandarin speaker. I don't know why you need to stress "taught" or "believe" so much. I'm not taught by anyone, nor believe that dialects are mostly similar. I experienced it. Most dialects, Cantonese included, use the same Chinese character system. In writing, we would use either simplified or traditional character sets. Note I'm only talking about modern communication.
> could be read and understood without deep knowledge of the actual language and world of the writer, resulting in, at best, comical translations
I don't know who you are talking about. I wouldn't say that at all. Chinese students would spent 1/3 of their high school studying complex ancient Chinese languages (文言文). Nobody would say it's not deep knowledge - it's so damn hard.
> could be read and understood without deep knowledge of the actual language and world of the writer, resulting in, at best, comical translations
I don't know who you are talking about. I wouldn't say that at all. Chinese students would spent 1/3 of their high school studying complex ancient Chinese languages (文言文). Nobody would say it's not deep knowledge - it's so damn hard.