“Ethiopians, the metzius is that there’s no evidence or reason to suppose they have any Jewish origin at all.”
We don’t posken by genotyping. All the reasons you list are why they need geirus. The fact that they claim to be Jewish and have been for as many generations as they can recall is what makes it a safek.
I’m not saying anything you’re saying is wrong. But, I don’t think any posek would tell you to take it out to its extreme and sell chometz to an Ethiopian Jew (pre-geirus). Personally, I believe they were Christians who re-invented Judaism based on the “Old Testament” as well, but we’re still choshesh for the slight possibility that this isn’t the case. The genetics brings nothing to the table, halachically-speaking.