Elder Esaias, this is a masterpiece of scripture interpretation.
At first I wanted to protest that the letter of Romans was written to the Church, and therefore your interpretation of gentiles being the northern scattered tribes could not be.
But after following all your references from
Genesis 48:19 (Ephraim becoming a multitude of nations) to the references in Hosea about scattered backsliding Israel being regathered to the Lord, I believe your interpretation is correct.