See, this is a perfect example of what I don’t get. When both the book of Acts ch15 and Galatians ch2 claim that James, along with John and Peter, were in theological agreement with everything Paul taught…how can you state that there was a theological difference between James and Paul?!
I sort of hate to do this since you can't respond, but the Epistle of James does not contain anything that is clearly a difference with Pauline theology, but that's because it deals almost entirely with proper behavior and ethics in this world and contain no eschatology, no ecclesiology, and other than an acceptance of Jesus as the Christ with no details of what that implies it has practically no Christology. Indeed, other than accepting that Jesus was Christ and that he would return, there's nothing in James that would contradict rabbinic Judaism. I'll go even farther than that. I don't think there's anything in James that contradicts the Qur'an since Islam accepts that Jesus was the Messiah but rejects that he (or anyone else) was literally the Son of God and that is a claim not made in James.
As for Acts 15 and Galatians 2, since Luke-Acts was clearly written by a Pauline Christian and pretty much every one agrees Galatians was indeed written by Paul, so it's not at all surprising that they'd share very similar theology, but that couldn't prove that they shared the same theology as James. At most they'd show that the Paulines asserted that there were no significant differences, but I don't even see that. While both passages show that James did accept that Gentiles could be Christians, they don't show James accepted Paul's belief that Jews were no longer bound by the law of Moses. Indeed, Galatians 2 indicates that at the least the followers of James thought Jews still had to keep the law of Moses, even if James himself conceivably might not have thought that.