If you look it up character by character in a Mandarin dictionary, you can interpret it. And if you look up the Cantonese sentence character by character in a Cantonese dictionary, you can interpret it.
No, neither pronunciation nor meaning are fixed, and phonetic drift in general is faster than semantic drift, i.e. people from different parts of China are more likely to ascribe different pronunciations to the same character than different meanings, although the latter is not uncommon. Canjobear above gave two examples where the Mandarin meaning has changed relative to the Classical Chinese meaning.
When people want to write something for which they don't know a character (either because none has ever been defined or they just haven't learned it or the pronunciation differs based on context and they feel this should be reflected by using different characters) they can create a new one based on whatever makes sense to them. The character 佢 was chosen because the word sounds similar to that written 巨 in Cantonese. Nowadays so many more people are literate in Mandarin that they're likely to substitute a character whose pronunciation in Mandarin is similar to what they want to write.