The prophets in Ephesians 2:20 were those mentioned in 4:11, not Old Testament prophets. Also 1 Cor. 12:28. Ministry gifts for building the church. I agree with your bracketed comment.
The promise to Abraham of blessing to the nations was not solely conditional on those nations blessing Israel. 12:3 adds that “all peoples on earth will be blessed through you”. This unconditional promise is repeated in 22:18 where there is no mention of a conditional promise. As far as a New Testament understanding of the promise goes, we have Paul’s 2nd temple exegesis of “the seed” in Galatians 3, which completely bypasses the need to be part of a restored Israel, and jumps from Abraham to his seed as all who belong to Christ - Jew/Greek, slave/free, male/female. There is no supra-Jewish identity. Clearly, Markus Barth was mistaken.
The “new man” comprised of course Jew and Gentile. The law needed to be abolished to let Gentiles in as well as letting Jews out. If Jews had not been let out, they would still be under the curse of the law.