When reading a book or a newspaper, what language do Cantonese people read in? Do they read in mandarin or Cantonese? Or do they subconsciously translate it into colloquial Cantonese since most words have a direct facsimile from mandarin? Also how do less literate people handle understanding Cantonese songs, which is sung in standard Chinese using Cantonese words? Finally, how do Cantonese schoolchildren learn in school?
When reading a book or a newspaper, what language do Cantonese people read in? Do they read in mandarin or Cantonese?
In Cantonese, of course.
Cantonese and Mandarin are not mutually intelligible, mainly due to a difference in their phonology. They share the same written language but each has its own set of pronunciation of the characters.
Or do they subconsciously translate it into colloquial Cantonese since most words have a direct facsimile from mandarin?
Cantonese can be written verbatim, but many of the characters are not regularly used. Instead, a non-verbatim formalized written form (same as Mandarin written form) is adopted. When a Cantonese speaker reads aloud a piece of writing, s/he uses Cantonese pronunciation. You, as the listener, will be able to tell that s/he is reading from a text, and not speaking colloquially.
Also how do less literate people handle understanding Cantonese songs, which is sung in standard Chinese using Cantonese words?
It is actually not that difficult, since most of the content words are the same, whether spoken or written. The words that are different from the spoken form are mostly function words.
Finally, how do Cantonese school children learn in school?
Cantonese school children learn in Cantonese. When they learn to read and write, they learn the standard Cantonese pronunciation of the written characters.
I have included a link here if you are interested in reading more about the Cantonese language. https://en.wikipedia.org/wiki/Cantonese
You can also google "History of the Cantonese language" for more information.
I think it is important to separate the written languages and spoken languages here. The Latin alphabet, for example, is used by many spoken European languages, yet almost nobody nowadays would say "I speak Latin" because the written language is simply a writing system with a fossilized name from its roots. The same can be said about the Chinese writing system, especially in older times. For example, 身体 means "body" in both Japanese and Chinese, yet Japanese people would only read it with their specific pronunciation ("shin tai") and Chinese people would use theirs ("shen ti").
A more complicated example is how this sentence is written in Chinese to mean "My name is Shawn.":
And this sentence in Japanese meaning the same thing:
Both technically have Chinese characters in the sentence, but you can see the differences become more extreme. With Cantonese, you typically don't have as dramatic of differences given they don't have syllabaries like Japanese, but consider this example for "what is this?" in Mandarin:
這個是什麽? (sounds like: "zhe ge shi shen me")
And in Cantonese:
呢個係咩? (sounds like: "li go hai me")
The pronunciations and characters differ radically despite the sentence meaning essentially the same thing.