Rav Yosef’s Psak is not binding on me as an Ashkenazic Jew. I will follow the Psak of the Ashkenazi Gedolim referenced earlier, namely, that Ethiopians should undergo Geirus L’chumrah. The fact that if they are really Jewish they would therefore be Sefaradim would have no bearing on the Psak which I should follow.
Beis Shamai’s Psak that the child is not a Mamzer would have no relevance for members of Beis Hillel, despite the fact that the child’s family were members of Beis Shamai.