Earlier quoted context omitted.
It's not clear to me whether it's one of the _least_ complex languages (there are various warts I'd get rid of if I were in control), but it's certainly not more complex than English. The "many pronunciations for one kanji" thing isn't a problem in practice, because 1) most kanji have _1_ on'yomi (Chinese) and _1_ kun'yomi (native) pronunciation, with it being obvious which one to choose from context, especially beca…
This is just not true let's take something very basic, the kanji 人 has two on'yomi pronunciations, jin and nin and there's no way to tell which one to use without knowing ALL of the words it will be used in 日本人 - nihonjin, so far so good 三人 - sannin, okay, I guess there's two pronunciations 二人 - futari, huh? 一人 - hitori, wat so please tell me how I am supposed to know 人生 (jinsei) from 人気 (ninki) from 恋人 (koibito) fro…
Your question about the reading, though, has quite a simple answer. It's pretty obvious how to read all the examples given when you look at them as words, not a single character. Kanji does not exist in a vacuum. 人気 is "ninki" and 大人 is "otona" - no other way around it. Combination of those characters, usually, have one and only reading. To tell the truth, in English, people are unconsciously reading words as a whole, too.