As we don’t have an official or authoritative body that determines “proper” English usage as other languages do, appealing to a dictionary strikes me as a mite better than prescriptivism or pedantry, though I don’t think was your intention either.

> Is it equally accepted for "peoples" to be possessive and "people's" to be plural?

That’s entirely unrelated and uncontroversial; one is the plural of a “people,” as in multiple distinct groups of folks with shared culture, nationality, or other traits, whereas the other is the possessive form of a word that is already plural, so I’m not sure if that’s a red herring or if you’ve actually seen such incorrect usage being advocated for.