When the noun is used with a numeral, you can use either plural or singular verb.
Yes, I guess so. «Шесть людей» sounds quite colloquial (and in general it sounds worse than «шесть человек»).
Челове́к is a strange noun that is normally not used in plural (the normal plural form is лю́ди ‘people’), so it doesn’t follow the usual rules.
The (non)pluralization lives is wrong, but yes, "Here live six people" should be fine. Course creator is not a primary English speaker as I understand so a lot of valid forms are rejected - I suppose this is more common phrasing around where I live and less so in the Queen's English or whatever the course wroter learned.