As for ひと・びと, those are kunyomi and you are better off guessing when its just better to use kunyomi. Youll be able to guess when to use rendaku as time goes on. Also じん is pretty much always used when describing people from a certain country. Can’t think of an example where it isn’t, but I’m not dumb enough to talk about japanese in absolutes.
That mnemonic actually works for a couple of words that spontaneously came to my mind except for one:
日本人 is にほんじん and I rather associate ninja with a japanese person than a genie. It works for for others though.
アメリカ人 is a foreigner (from japanese point of view), foreigner and genie kinda works.
Maybe I just thought of the only example where it doesn’t work, I don’t know
I like the ninja idea. I use the person drinking gin to remember jin. So 殺人 (さつじん) is murder via poisoned gin. Or 詩人 (しじん) is poets like to drink gin (especially Charlie Sheen).