DY,
1. Read it again. Rav Moshe was talking about before giur. He said that they should be mekareved and then undergo giur l’chumra. He said that some people do not want to because they are black.
2, The Ethiopians in question have undergone conversion or are children of converts.
Dull & Yitczchok, for a non-Jew the night follows the day. Therefor, if a non-Jew keeps Shabbat but soes melacha on Motzash he is in the clear.
Ran, true but he cannot dance a two weddings. If they are safek mamzerim there is at least a safek if he is Jewish. As I have previously posted, stam yaynam is rabbinic so the we are lenient. One who says differently is disagreeing with Chazal and all Savoraim, Gaonim, Rishonim and Achronim who came before him.