I use the NT interp of the OT. If Paul doesn't make anything out of Keturah, and does make something out of the non-ordinary conception, then I go with that. I don't go with you with your excessive familiarity with every OT genealogical corner you can think of and with your ignorance or unwillingness to comment on Gal 4:23.
many people avoid what Paul says about the OT. You do it your way.
Last I checked the Christian message was the NT and how it quotes the OT.
And, one more time, what matters is the conclusions of 3:29 about who is an inheritor with Abraham. It is in Christ. I guess then, that you are not, because you say nothing of it, seem to know nothing of it and nothing in that verse seems to register anywhere in what you say.
You use an NIV, that's a major contributor to your ignorance. Galatians 4:23 KJV... really simple to a blind wild hawg: Abraham sired them both, but only Isaac was the promised seed OF ABRAHAM, btw. Paul said Abraham had TWO SONS in the very prior verse: Galatians 4:22 KJV. Paul did NOT say Isaac was God's direct progeny, and Ishmael was Abraham's progeny. What is your incentive to extrapolate more out of that simple understanding other than to cast shadows on the faith of Abraham?
Where's the Christian message before Christ came onto the scene? That's the part you don't get. Jesus was the fulfillment of Abraham's faith. The NT began with "The book of the generation of Jesus Christ, the son of David, the son of Abraham" (Matthew 1:1 KJV). But, generations means nothing to you, speaking of endless genealogies. The last I checked, 4,300 years of human history, manifest in the divine arrival generation of Jesus, "seventy and sevenfold" inclusive generations from God (Genesis 4:24 KJV, Luke 3:38 KJV-Luke 3:23 KJV), was not infinity. You do know what infinity is, right? Endless? Look that word up and see if infinity ends with 77.
So, you can take your accusation of my ignorance of infinity back to that curator of some museum in Turkey. You've not even answered your own question: Was Isaac God's direct progeny, or was Isaac Abraham's progeny? You just shift from foot to foot stirring up dust, and fluffing your feathers. Please forgive me that I'm totally underwhelmed with your majestic missionary status, and alleged academics in theology. You're the one who got 'missionized' in Turkey.