That "Spiritual Isreal" thing was a disaster waiting to happen, did the first century consider Paul a profit? I never got that impression from him, he uses very expressive allegories but he mostly connected old testament concepts to messianic faith. Spiritual Isrealites would be an entirely new concept that was not taught by prophets.
The prophets taught the other nations coming into subjugation of Isrealites, and that their inheritance was basically them being absorbed into Isreal customs under the messiah and not being killed off.
There was a mixed multitude of gentiles that came into Isreal out of egypt (who believed under the old covenant) and they could worship with Isreal in a specific order, so it never was a foriegn concept. There was always a prophetic distinction between Isrealites and gentiles.