Addnedum. Crossan argues that modern scholarship shows that Paul did not preach to Jews or primarily Gentiles but "god-fearers," members of other nationalities that were attracted to Judaism. If circumcision were not the requirement to convert, they would have become Jews. They worshipped and studied with the Jews. It makes a lot of sense to me. If Paul were preaching to Gentiles, why would he even mention Jewish custom to teach Jesus. Paul's voice invented Christianity as we know it. Paul's preaching to Jews would not find a welcome audience.
If the quotes were from Peter or James, apostles known to be in Jerusalem, (Matthew would be a great example if the author were the apostle), your argument might have more sense. Paul was boldly going where no Christian went before. The average JW knows much more about 1st century history than you do. Scholarship is not your thing, though, is it?