Paul Johnson speculats that more Jews came into the Church than we has thought. Fact is that the Jews in the West owed less to the Jews of Mesopotamia than this win the Levant and Anatolia. In Alexandria, Rome, Gaul and Spain the doctrine of the Pharisees was less well know,and Hebrew less spoken, and so more open to the Gospel.
Fact is we have only a very limited idea of how Christianity spread during the first generation.
That is probably true, the diaspora paved the way for the spread of the gospel among Jews and then gentiles. However what Paul taught in Romans 9-11 is absolutely true and necessary. The majority of Israel did not believe, and that required the explanation and the caution-