"The olive tree is the place of privilege that was first occupied by the natural branches (the Jews). The wild branches are Gentiles who, because of the unbelief of Israel, now occupy the place of privilege. The root of the tree is the Abrahamic covenant that promised blessing to both Jew and Gentile through Christ."
Do you suggest that any biblical Christian would suggest that "On this rock I will build my Church" did not include the Jewish Apostles themselves?
Yes, we are familiar with Darby's theories. Quoting him does not really help.
The question is, how do you distinguish between the remnant in the Church and some other remnant without being entirely artificial? E.g., why the need to divide “all Israel” up into distinct chunks?
Do you suggest that any biblical Christian would suggest that "On this rock I will build my Church" did not include the Jewish Apostles themselves?
Absolutely not. It is my argument that every Jewish believer in Jesus Christ is the remnant of Israel (“all Israel” if you will) and is being saved during the time in which we live. They are the Church, along with their gentile brethren. Natural and wild grafted into one root, Jesus Christ.
In fact you are making the non-dispensational case very well.