I think taking sides in this debate is pointless. Languages are always as difficult as far they are from your mother tongue or other languages you know.
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.
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.