You can also guess Kanji readings just by looking at them. The largest class of Chinese characters consists of the "phonosemantic compounds", which consist of a phonetic part (how to say it) and a semantic part (what it means). Over 90% of Chinese characters fall in this class, but I'm not sure what rate you'd encounter them in Japanese. You can start doing this fairly well once you know... I'm not sure... probably in the vicinity of several hundred characters.
Several different readings is a bit of an exaggeration, it's more common to just have a kun'yomi and on'yomi reading, and on average I think the number of readings you'd know for a character is between one and two.
Purely from personal experience, I hear adult native English speakers (including myself) mispronounce words all the time! It's a common mistake to pronounce a word as if it were borrowed from e.g. French when it was really borrowed from Greek or Latin. So if you want to know how to pronounce "ch", you might say /ʃ/ if it's French and /tʃ/ if it's English. This seems to me like another variation on the same problem we have with reading Japanese, where characters have Japanese and Chinese readings.
Several different readings is a bit of an exaggeration, it's more common to just have a kun'yomi and on'yomi reading, and on average I think the number of readings you'd know for a character is between one and two.
Purely from personal experience, I hear adult native English speakers (including myself) mispronounce words all the time! It's a common mistake to pronounce a word as if it were borrowed from e.g. French when it was really borrowed from Greek or Latin. So if you want to know how to pronounce "ch", you might say /ʃ/ if it's French and /tʃ/ if it's English. This seems to me like another variation on the same problem we have with reading Japanese, where characters have Japanese and Chinese readings.