Rafiki is not in the first noun class. Rafiki (class 9) can be both singular and plural; marafiki (class 6) is plural (friends). Although in class 9, adjective agreements for rafiki follow class 1/2 (animate beings). So a good friend is 'rafiki mzuri' rather than 'rafiki nzuri'. But possessive agreements are based on class 9. So 'my good friend' is 'rafiki yangu mzuri' and 'my good friends' is 'rafiki zangu wazuri'. I'm not sure how 'marafiki' would be used like this, it would either be 'marafiki yangu wazuri' (class 6 possessive agreement) or 'marafiki zangu wazuri' (class 10 possessive agreement). I'm not even sure if they'd still maintain the same adjective agreements of wa-.... anyways, best to stick with rafiki for singular and plural
It has, but since Swahili doesn't have articles, both would be right.
But this phrase is hella weird, I don't think that anyone would write this, unless this person is testing the limits of the app.
Please, don't annoy the staff with this kind of thing. Only report something when you accidentally uses an answer that isn't accepted yet, not when you are testing the limits of duo.