The Ethiopians practiced their form of monotheistic “Judaism” in Ethiopia. I’ve never had a chance to see the tshuvos on the matter (it would be nice if someone would give a mar’eh makom or post a link), but based on their practices it seems the only way to consider them as being originally Jews is to deny Torah sheb’al peh. It seems much more likely that they were originally a Christian sect that kept a very Biblical religion. But they kept their religion with much mesirus nefesh and were hated by the local Christians, which makes it unlikely that there was much intermarriage between them.
The bigger issue that everyone is ignoring is that these are major halachic questions which should be answered by a posek who has all the facts, not in the YWN coffee room.