Yes, they are a distinct people group. But there is only one way to be "Abraham's offspring" and that is to be in Christ by faith. There is one shepherd, one flock. There is one faith, one baptism. The issue of how this great mystery (that the Gentiles are included, and the two become one in Christ) plays out was a major source of controversy but the question was resolved first by the Jerusalem council, and later by Paul's Epistle to the Galatians. Gentiles don't have to come to Christ via Moses because no one has to come to Christ via Moses, not even ethnic Israel.

It is not eisegesis to repeat Paul: "ALL the promises are Yes and Amen in Christ." In context he is referring to the promises given to Abraham. "If you are in Christ, you are Abraham's seed indeed, subject to ALL the promises."

Whether Romans 11 is referring to a future mass conversion or something that is ongoing since the days of the Apostles is a matter of some debate.