The problem is, earlier in this argument Paul's rebuke of Peter for withdrawing from Gentile believers was equated with Paul's willingness to indulge believers who felt they should continue to observe the cultural trappings of the Law. But these are not the same issues.
To withdraw from fellowshipping with fellow believers over race is racism. But on cultural issues Paul indicated it was necessary to observe some of the externals of a culture if one is to reach a different culture, particularly with young believers who were raised up in it.
In one case Paul warned that immature believers, such as those who would not eat food sacrificed to idols, should be respected by not practicing such eating in their presence. On the other hand Paul warned that the Gospel would not be effective if cultural adornments were not properly respected in that culture while the Gospel was being preached--two different issues.
On the matter of going to Jerusalem it is clear that Paul was warned by God that he must go to Jews and Gentiles in the face of suffering. The prophecy warning Paul about Jerusalem was a reflection of the prophet's concern for Paul suffering, but Paul had already know that he would suffer and was willing to obey God despite that.
2 Cor 12.9 But he said to me, “My grace is sufficient for you, for my power is made perfect in weakness.” Therefore I will boast all the more gladly about my weaknesses, so that Christ’s power may rest on me. 10 That is why, for Christ’s sake, I delight in weaknesses, in insults, in hardships, in persecutions, in difficulties. For when I am weak, then I am strong.
As for the prophet's ignorance of Paul's call to suffer it is clear that in his warning Paul not to go he did not know what Paul already knew, that he must go to Jerusalem and suffer in the process. It was the prophet who erred--not Paul. It wasn't really error so much on the part of the prophet, but concern for Paul with a lack of information about what God had told Paul.
Paul, while he was in Corinth, proclaimed the Gospel to Jews resistant to his message. But we read this...
Acts 18.9 One night the Lord spoke to Paul in a vision: “Do not be afraid; keep on speaking, do not be silent.
So Paul was following his call to reach both Jews and non-Jews with the Gospel. And reaching the Jews required that Paul find acceptance of the Apostles in Jerusalem, as well as preach the Gospel in Jerusalem, despite his call to reach the Gentiles. This was particularly disturbing to Jews who under the Law had been told that Gentiles were pagan and should be ignored.
Your problem is you just don't believe the record of Scriptures and what Paul had said about these things. Jesus had told the original 12 Apostles to begin their testimony in Jerusalem even though they were also warned that they would be persecuted in the process. And Jesus himself died there. Paul is simply following in the steps of the original 12 Apostles in persisting in going up to Jerusalem in the face of persecution.
Acts 20.22 “And now, compelled by the Spirit, I am going to Jerusalem, not knowing what will happen to me there. 23 I only know that in every city the Holy Spirit warns me that prison and hardships are facing me. 24 However, I consider my life worth nothing to me; my only aim is to finish the race and complete the task the Lord Jesus has given me—the task of testifying to the good news of God’s grace."
Acts 21.13 Then Paul answered, “Why are you weeping and breaking my heart? I am ready not only to be bound, but also to die in Jerusalem for the name of the Lord Jesus.”
Acts 23.11 The following night the Lord stood near Paul and said, “Take courage! As you have testified about me in Jerusalem, so you must also testify in Rome.”
Paul and James agreed that young Christians raised in Judaism should not be encouraged to blatantly defy the cultural and moral values in Judaism so that they could remain "like the Jews to win the Jews."
1 Cor 9.20 To the Jews I became like a Jew, to win the Jews. To those under the law I became like one under the law (though I myself am not under the law), so as to win those under the law.
Acts 15.20 Instead we should write to them, telling them to abstain from food polluted by idols, from sexual immorality, from the meat of strangled animals and from blood.
And those young in their conversion had trouble distinguishing between some of the prohibitions of the Law from moral prohibitions in Christ. And Paul wanted to allow for time to mature before trying to clarify these more-difficult matters.
Rom 14.1 Accept the one whose faith is weak, without quarreling over disputable matters. 2 One person’s faith allows them to eat anything, but another, whose faith is weak, eats only vegetables.
When Paul went to Jerusalem the last time it was in the stream of his regular visits there, sometimes to bring financial support to Jewish believers, and sometimes to coordinate his ministry to the Gentiles with those ministering among the Jews. But he was aware of his proximity to martyrdom as his ministry approached its later stages. And he was no doubt aware that Jesus had ended his ministry in Jerusalem.
It may be that Paul prompted Jewish believers in Jerusalem to maintain a semblence of obedience to the Law out of respect for those in Judaism. Though many of them were not yet clear on how much liberty the Gospel afforded them they were nevertheless encouraged to maintain respect among those still in Judaism.
So following the Law externally had the practical purpose of not provoking those whose culture required Jews to remain externally obedient to the Law. Jewish believers may have been ignorant concerning the division between OT and NT requirements, but it was essential that they know Christians do not intentionally disrespect customs that do not have anything to do with spiritual uncleanness.
This is a lot, I know, to take in. But over-simplifying it by "attacking Paul" as a "lapsed Christian" doesn't work for me. Undoubtedly Paul was as flawed as we are. He got hot-headed insulting a high priest albeit naively. But the Scriptures appear to support Paul's last trip to Jerusalem, as well as his counsel of Jewish believers in Jerusalem. We should take care before questioning Paul's motives.