That is an okay point to note. And the blending of the tribes into bloodline of the surrounding nations is a decent observation along with the idea that God was covering an obligation to the ten lost tribes.
God had no obligation to Gentiles. None. As a matter of biblical fact God has no covenant with Gentiles. None.
God has covenant with Abraham and his seed and God is obligated to Abraham and his seed as per covenant and later through prophecy. God's attitude towards Gentiles is as follows:
17 All nations before him are as
nothing;
And
they are counted to him less than nothing, and vanity.
Isaiah 40:17.
The bible is a record of the Hebrew people written by Hebrews to and for other Hebrews and this included the New Covenant writings from Matthew to Revelation. By disrespecting Gentiles and seeing them as "vanity" shows the great disdain God has towards Gentiles.
God has respect towards Hebrews, a people, a race, an ethnicity if you will, who is in covenant with God. Therefore, because of God's Promises He will do all things necessary to fulfill every promise He's made to the Hebrew people beginning with Abraham, then to Isaac, then to Jacob, and finally Jacob's children also known as the children of Jacob/Israel. And yes, there may be some Jew-Gentile offspring but for the most part because of covenant Jews married Jews. It was through disobedience, conquest and slavery, rape, concubinage, etc., that Jews - both male and female - found it necessary, even for survival, to couple with Gentiles, but being that the Jews knew theirs was an exclusive covenant that included no Gentiles. This is why Jews took issue with the offspring in Samaria after the Assyrian conquest which a remnant population of Jew-Gentiles lived that 'pure' Jews took issue and were hated because these jews brought shame and uncleanness into the fold.
So many of the nations/places surrounding Judea heard the gospel and responded to it while being of some Israel bloodline descent.
Very far and few did offspring from Jew- Gentile couplings became Gentile in life and living but were persuaded as Jews to live in Gentile lands as Jews trying to live according to the Law of Moses. These were called Hellenized Jews, not Hellenized Gentiles. Purely non-Hebrew Gentiles did not respond to any gospel because theirs was a Godless idol-worshiping false religion. My position is that there are no Gentiles in any of the salvation Hebrew covenants as the first covenant is with Abraham and his seed; the Mosaic Covenant is with the children of Jacob/Israel, and the New Covenant is with the House of Israel and Judah. This is the bottom line. Had God included non-Hebrew, non-Abrahamic seed Gentiles in any of the covenant He would have said so and been clear about it, but there are no Gentiles in the Hebrew covenants. None.
What Jesus' interaction with the Samaritan woman specifically and Samaritans generally teach me is that as long as one is seed of Abraham no matter the Jew-Gentile DNA dilution in offspring, they are in covenant with God.
Looking at history from 586 BC when Cyrus allowed Jews to return to their land, those that did return were a "remnant" (10%) which also included a minority of the ten tribes that were taken captive by Assyria and when Assyria was conquered were taken captive by Babylon. During Israel's captivity in Babylon Judaism sprung up. There was conscious effort to remain obedient to Torah and Torah Law even without a Temple and yet their religion centered around their Feasts and Holy Days and observances. After 586 BC time allowed for Jew-Gentile offspring to continue in general so that by the time Jesus arrived there were some Samaritans who were Jew-oriented that lived in Israel yet mostly in Samaria, the one-time capital of the ten northern kingdom tribes, and Jesus interacted with these Jew-Gentile offspring and blessed them. The Roman centurion comes to mind. Being a God-Fearer and one who "loved our nation" and either built by himself or with assistance and/or even funded the building of a Jewish synagogue understood the Old Covenant Scripture that spoke of Messiah and easily recognized Him in Jesus as well as knew Jewish protocol in Jewish-Gentile dealings about entering a Gentile habitation.
This Jew-Gentile offspring continued even to this day, especially knowing two things:
1. God made covenant with Abraham and his seed. Jew-Gentile offspring remained Abraham's seed no matter the DNA dilution. God has covenant with Abraham and his seed.
2. Everyone in the bible and even to this day who is said to be Gentile or proselytes or God-Fearer, who sees themselves as Gentile whether or not they are aware of Hebrew blood in their family line who is born-again and saved just may have Abraham's DNA in their family lines and being the covenant is with Abraham's seed and NOT with non-Hebrew Gentiles, everyone who claims to be Gentile and is born again has Hebrew DNA in their families. Although reasonable, this cannot be proven but neither can it be disproven.
Many of these people became Christians and thus fulfilled the promise of gathering those who were scattered. They were gathered into eternal life per Dan 9:24. The bad news is that most did not come and were destroyed, as found in Zephaniah -- of the land under utter devastation. Only the remnant were spared as per Isaiah 10:20-22 and most of the others died. Sure some non-Christians survived but the survivors continued the evil doctrine of the Pharisees and preserved the corruption of doctrine prevalent in the first century -- but this no longer was the ancient teachings through Moses.
The Pharisees did not have an evil doctrine. In evidence Jesus held more closely to Pharisaic teaching than to Sadducees doctrines. Both Jesus and Pharisees believed in eternal life, angels, resurrection, and several other chief doctrines they held. The issue Jesus had with the religious order in Israel is that they elevated and exalted the traditions of the elders over the Hebrew Scripture (Word of God) and this is clearly seen in Jesus condemning this very thing they did. And why the religious leaders did challenge Jesus in the Torah Law the breach in understanding lay in this one simple fact: The religious leaders only knew the letter of the Law and Jesus for the most part taught the Jews the spirit of the Law. Acts 6 records that a "great company (over 51%) of the priests were obedient to the faith."
Pharisees and Sadducees developed while in Babylonian captivity and were of good service to the people except in time there was a tendency of elevating the traditions of the elders above Scripture. The same thing exists in Christendom today where "Christians" elevate doctrines of men - whether commentaries, theology book authors, and even the teachings of men such as Calvin, Luther, Jack Hayford, Billy Graham, Charles Stanley, Augustine, etc., above the very Word of God. The Pharisaic tradition lives in Christendom today! Even here, online.
What is corrupt is Gentile doctrine and theology. Once Gentiles unmoored true, biblical Christianity from its Hebrew roots after the destruction of the Jewish Temple they created a new religion and one without the Christ of the bible.