[wst_]

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.

[iopq]

Great, so what is 大人気?

if it's 大人気ない it's probably childish, but if it's not in that context, it's probably very popular

You need to finish reading the whole SENTENCE to be able to pronounce it

[wst_]

Not sure I agree with that. It's not difficult to notice one has "nai", other don't. Also when you read it in actual sentence you can immediately notice that something is wrong when you choose the wrong reading. Subsequent words don't add up. I am not saying confusion doesn't occur. I'm just saying it's not as big as people are painting it.

[iopq]

It's not always the case it's followed by nai

大人気なくてもいい means it's okay to be childish