[The fullness of the gentiles is facilitated by the return of the Ten Lost Tribes that intermingled with the nations and were virtually indistinguishable from the gentiles... that’s why it says all of Israel shall be saved, that’s all twelve tribes... as James says in his epistle, to the twelve tribes...]
I think it is important to recognize where Apostle Paul is getting his information from and it’s obviously from the Old Covenant writings.
““Yet the number of the children of Israel Shall be as the sand of the sea, Which cannot be measured or numbered. And it shall come to pass In the place where it was said to them, ‘YOU ARE NOT My people,’ There it shall be said to them, ‘ You are sons of the living G d.’ Then the children of JUDAH AND the children of ISRAEL Shall be gathered together, And appoint for themselves one head; And they shall come up out of the land, For great will be the day of Jezreel!”
Hosea 1:10-11 NKJV
Contrast that to
““And it shall come to pass in the place where it was said to them, ‘You are not My people,’ There they shall be called sons of the living God.””
Romans 9:26 NKJV
Here we have a MAJOR ISSUE because G d divorced the North ten tribes and the law of Moses says that so long as the first husband is alive, the divorced woman cannot remarry. So the Northern Ten Tribes by blood are divorced and intermingled with the gentiles. Jesus the Messiah comes and says accept me as your bridegroom enter into Covenant with me. There’s a problem in the mind of the Jew. Gentiles have divorced Northern Ten Tribe Blood and so how can the gentiles receive salvation? G d can’t enter into Covenant with those He divorced.
“Then I saw that for all the causes for which backsliding Israel had committed adultery, I had put her away and given her a certificate of divorce; yet her treacherous sister Judah did not fear, but went and played the harlot also.”
Jeremiah 3:8 NKJV
But Apostle Paul explains further.
“For the woman who has a husband is bound by the law to her husband as long as he lives. But if the husband dies, she is released from the law of her husband.
Therefore, my brethren, you also have become dead to the law through the body of Christ, that you may be married to another—to Him who was raised from the dead, that we should bear fruit to G d.”
Romans 7:2, 4 NKJV
In other words on Mt Sinai it was the Angel that spoke and that made a Covenant with all the twelve tribes and the gentiles present, or Jesus. And now Jesus the first husband DIED but what more, He rose again. So now the divorced Ten Tribes are free from the law of the first husband.
But we enter into the New Covenant not by bloodline John says
“who were born, not of blood, nor of the will of the flesh, nor of the will of man, but of G d.”
John 1:13 NKJV
Therefore anyone can come in.
I think what is throwing us off is the choice of the word Israel rather than the Judah or Jews the southern kingdom in the verse you quoted. In a sense all twelve tribes are blinded, the Jews (Judah and Benjamin) who refuse to accept their own messiah and the North Ten Tribes who may have lost their heritage due to intermarriage.
But the next verse says ALL of Israel will be saved.
And G d’s promise to Abraham was that in him or in his SEED (referring to Christ) all the nations of the earth will be blessed. This happened when Jews and gentiles accepted Christ. The fullness of the gentiles is essentially all the nations that were divided according to Deuteronomy 32:8 to the the sons of G d coming BACK under the dominion of G d.
And this was partially facilitated with the intermarriage of the Northern Ten Tribes with the gentiles who had a strong affinity to G d, some of whom still knew their heritage in their tribe but were alienated by the Jews because they were G d’s divorcees.
I’ll end with the heart of G d longing for His divorced Efraim
““How can I give you up, Ephraim? How can I hand you over, Israel? How can I make you like Admah? How can I set you like Zeboiim? My heart churns within Me; My sympathy is stirred.”
Hosea 11:8 NKJV
G d had a plan and the heavenly hosts were unable to understand how He could take back those He divorced.