What I can say is that Peter thought that Paul was a genuine Christian (2 Peter 3:15). James, the brother of Jesus, as well as all the apostles in Jerusalem believed that Paul was a genuine Christian (Acts 15). All, or perhaps better to say, nearly all of the early Christian writers believed that Paul was a faithful Christian (including Clement of Rome, Justin, Origen, Irenaeus, Polycarp, Hippolytus and many more who quoted Paul as an inspired writer of Christian scripture. One heretic named Marcion did question Paul’s apostleship.). Paul believed he was a genuine Christian. There are no important ancient voices from the first or second century who questioned that Paul was a genuine Christian.
So, who are these supposed scholars who doubt that Paul was a genuine Christian? What are their criteria? By the way, NT Wright DEFINITELY believes that Paul was a genuine Christian. I do not know why his name came up in your question.
OK, some people doubt that Paul was a genuine Christian. So what? They can think what they like, but God thinks that Paul was a faithful Christian because God ultimately chose what ended up in the Scripture, including Paul’s writings. He used human beings, and most specifically his apostles, including the apostle Paul, to decide what was inspired. If God caused Paul’s writings to end up in the New Testament, then who am I to doubt God on that? I believe that all Scripture is inspired by God.
Sure, it is very helpful in order to interpret Paul, but also John and James and the writer of Hebrews to learn as much as possible about the second temple period of Jewish history. No one denies this! What I can say is that it was not the second century church fathers who told the story of the conversion of Paul. We have this story from Paul himself in Galatians, which was written in the 50s AD, and we have the story from Luke, who wrote it in the 60s AD, so the idea that this story was created by the second century church fathers is nonsensical. I agree that “Paul was establishing a new way of living for Jews and Gentiles–a way that they could live in the space made available to all by the resurrected Messiah.” This is a correct statement. In this Paul agreed with Jesus, John, Peter, and the other biblical authors. This is consistent with many dozens of Old Testament passages as well! (Isaiah 42:6, Isaiah 49:6 for example). Paul definitely did not invent the idea that in Christ, God would bring together both Jew and Gentile, and he did not invent the idea that the Law would be replaced by Christ and by salvation by grace. I can provide dozens of OT passages to support this as well if you like.
As for the Eisebaums, I suppose you should read the book if you like and try to understand what they are trying to say, but please do not throw away the obvious, which is that the entire Bible is inspired by God and that it was God who was responsible for the inspired things that Paul wrote. Paul was most certainly a faithful Christian, no matter what the Eisenbaums say.