Regarding the second part of your post about the Greeks following the Jewish law, I’m not sure whether you mean the ‘national’ law of the Jews or the Mosaic Law, the religious law of the Jews. I will assume the later. I thought of several instances of what you say: 1) circumcision, 2) animal, or I will call them ‘typical’, sacrifices and 3) dietary ordinances. I will take the first one in this post.
There was a sharp contention in the early church about circumcision of Gentile converts. The contention was regarding the relationship of the Gentiles to the Mosaic Law and, perhaps, the necessity of the Gentiles becoming Jews (being circumcised) in order to receive salvation. Acts and Galatians mention this incident. Galatians deals extensively with Paul’s warning to the Galatians not to turn the Jewish legal system signified by receiving circumcision as it was attended by the principle of justification by works of law.
Acts 15:1 And some men came down from Judea and began teaching the brethren, "Unless you are circumcised according to the custom of Moses, you cannot be saved." 2 And when Paul and Barnabas had great dissension and debate with them, the brethren determined that Paul and Barnabas and certain others of them should go up to Jerusalem to the apostles and elders concerning this issue.
Galatians 2:3 But not even Titus who was with me, though he was a Greek, was compelled to be circumcised. 4 But it was because of the false brethren who had sneaked in to spy out our liberty which we have in Christ Jesus, in order to bring us into bondage. 5 But we did not yield in subjection to them for even an hour, so that the truth of the gospel might remain with you.
The conclusion of the issue was that circumcision was not enjoined on the Gentiles. The conclusion was not based on whether or not the Jews were a reigning nation. I'm not sure if there is a definitive answer about circumcision of Jewish followers of Jesus as Paul circumcised Timothy.
Paul does give a spiritual application of circumcision - applicable to all Christians.
Colossians 2:11 and in Him you were also circumcised with a circumcision made without hands, in the removal of the body of the flesh by the circumcision of Christ.
By the way, if I understand it right, I say peshat too (most of the time).