There is nothing in Scripture that even comes close to suggesting that James' address to his brethren supports that the Church is the Twelve Tribes of Israel. If we take such an approach then we must equally say that the entire Church are Corinthians, Romans, or Hebrews.
If there is neither Jew nor Greek, then there is no classification for the Church except that they are the Church.
God bless.
God chose Israel to be a kingdom of priests (Ex.19:6), and Apostle Peter repeats that same chosen concept about Christ's Church in 1 Pet.2:9. Thus God's Israel is... Christ's Church in final, irregardless that some among Israel refuse to believe.
In Galatians 3, Apostle Paul made it plain that the Promise by Faith is what Abraham was given and he believed, and thus all those who have believed as Abraham did have become the children of Abraham. That's about Christ's Church. The Promise by Faith was always first, prior to the giving of the law.
In Romans 11:1-5, Apostle Paul is specifically speaking of an elect remnant of the seed of Israel which God reserved, which is also about Christ's Church.
Rom 11:1-5
11:1 I say then, Hath God cast away his people? God forbid. For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin.
2 God hath not cast away His people which He foreknew. Wot ye not what the scripture saith of Elias? how he maketh intercession to God against Israel, saying,
3 Lord, they have killed Thy prophets, and digged down Thine altars; and I am left alone, and they seek my life.
4 But what saith the answer of God unto him? I have reserved to myself seven thousand men, who have not bowed the knee to the image of Baal.
5 Even so then at this present time also there is a remnant according to the election of grace.
KJV
That means a believing remnant of the seed of Israel, Christ's Apostles representing those.
In Ephesians 2, Paul declared the foundation of Christ's Church being built also upon the OT prophets of Israel.
So was James literally speaking of BELIEVERS on Christ Jesus when he addressed that to the twelve tribes scattered abroad? YES! Most definitely!
The majority of all 12 tribes were scattered through the countries according to God's Word, with only a very small remnant of the "house of Judah" (2 tribes) having returned to Jerusalem after the 70 years Babylon captivity (Ezra 2).