Ubiquitin,
1. It was definitely true regarding the Ethiopians. I do not know what the procedure will be regarding the Kaifeng community but it would seem to me that it should be the same as for the Ethiopians.
2. I do not know if anyone has even asked before you. If you have a strong desire to sell him chametz (assuming that you are expert in the procedure) I suggest that you ask your LOR. Regarding Shabbat, you cannot even ask a certified gentile to do a melacha d’Oraita except under extreme circumstances (e.g. you forgot to turn on the heat and are freezing). Even a shevut requires a tzorech mitzva, hefsed meruba or choleh sheain bo sakkana. You can learn some of the basic halachot of “amira l’akum” in Orech Chaim 307.