One of the questions asked is why do we only see the tribe of Judah in the NT and not the Northern Tribes? We can figure this out as we go through the biblical narrative. We do not have to make up things about the Northern Tribes being Gentiles. The bible tells us that there were three migrations from Babylon and Persia to Jerusalem after the time of Daniel. We see that when Daniel went into captivity, Babylon captured the Kingdom of Judah. During the time of Nehemiah, who himself was of the tribe of Judah as well, the Jews (Southern Kingdom) returned to Jerusalem. We do not see the Northern Tribes return to Jerusalem. This is why in my opinion we do not hear of the Northern Tribes in the New Testament period. In the scriptures we know that many of the Israelites were still among the nations. This doesn't just mean Judah, but Northern Tribes as well. As we know from History many were in Africa. How do we know this? We have the story of Philip and Ethiopian Eunuch. In my opinion the Northern Kingdom never returned. Correct me if I am wrong, but this is the logical analysis of the scriptures, and we do not need to twist anything by saying the Gentiles were the Northern Tribes. Contextually, every time we see Paul talking to other nations, he means other nations (Gentiles) besides Israel. Acts is the only places dealing with Hellenistic Jews.
There were Greek-speaking Jews of the Diaspora, who returned to settle in Jerusalem. To identify them, Luke uses the term Hellenistai. When he had in mind Greeks, gentiles, non-Jews who spoke Greek and lived according to the Greek fashion, then he used the word Hellenes (Acts 21.28)