Presented with choice between Christianity and Islam early in the 20th Century, the Abayudaya adopted Judaism. The Abayudaya are Jewish.
The Native American community is not a “nation” — it is an assembly and cohort made up of many nations with many formerly at war with one another.
While you are looking for bedrock authenticity, we Jews may enjoy both adoption and lineage in the construction of our identity.
In the feudal and medieval modes, lineage may be made to count for a great deal in the inheritance of power and privilege related to “purity” of some kind (as most kingdoms learn, genetic “purity” turns out badly pursued too well), but in the modern world, identity has much to do with what is in the heart and shared from it.
Now the sons of Israel journeyed from Rameses to Succoth, about six hundred thousand men on foot, aside from children. 38 A mixed multitude also went up with them, along with flocks and herds, a very large number of livestock. 39 They baked the dough which they had brought out of Egypt into cakes of unleavened bread. For it had not become leavened, since they were driven out of Egypt and could not delay, nor had they prepared any provisions for themselves.…
/// (Exodus 12:38).
Who in that “mixed multitude” that shared the experience undertaken by the sons of Israel would not come to be themselves considered Hebrew and Jewish?