I am sure God does know who His chosen people are and so can we from studying the scriptures. I think the problem you are having as do many is that you are confusing Gods spiritual promises and Gods national promises.
First) Jacob, also called Israel is whom from the twelve tribes originated which made up the nation of Israel. As promised in the scriptures, God has restored this nation to the land and it is only to literal Israel that the national covenant promises of God related to this land apply.
Second) One hundred years or so after the Babylonian captivity when the exiles returned to Jerusalem and the land of Israel the terms Jews or children of Judah" and Israel or children of Israel were used interchangeably by Ezra, Nehemiah, Jeremiah and in the NT by Jesus, Paul, and the other apostles showing that all the tribes were present and that Jews includes all the people or tribes of Israel.
Third) In the NT in the book of Galatians Paul speaks about justification through faith and points out that when Gentiles (non-Jews or those descended from Jacob/Israel), demonstrate Abrahams kind of faith and obedience, toward Christ they become ... Abrahams seed, and heirs according to the promise. Gal.3:29
When Abraham demonstrated complete faith in God, the LORD said... blessing I will bless you, and multiplying I will multiply your descendants as the stars of the heaven and as the sand which is on the seashore; and your descendants shall possess the gate of their enemies. In your seed all the nations of the earth shall be blessed, because you have obeyed My voice. Genesis 22:17-18
So in this way, in a spiritual sense only Abrahams descendants have been multiplied as the sand and stars because Jesus Christ descended from him, the nations have been blessed, and many have believed in Christ by faith, faith like Abrahams. I think you must be careful though with such language. I believe it's important to take the scriptures literally as much as possible when it is meant to be taken so. Yet, in this case I think the language is metaphorical or figurative because even if every person in the entire world was considered one of Abrahams descendants that still would not be as many as the grains of sand on the seashores or stars in the heavens, would it?
Finally, the NT always makes a distinction between the church (body of Christ) and Israel. I dont believe the scriptures ever teach that the church is spiritual Israel, has replaced Israel or that they are one in the same. Israel always means Israel and the people of Israel have not disappeared, but are in Israel as well as being dispersed throughout the world and are now known as the Jews.