Paul's silence in that letter to his converts in Galatia as to the decision of the Jerusalem Council forces the irreconcilable dilemma of saying either (1) that Luke’s account in Acts 15 of a decision reached in Paul’s favor at Jerusalem is pure fabrication, or (2) that Galatians was written before the Jerusalem Council.OR (3) Luke's account is factual, Paul wrote Galatians after the Council, and Paul had his reasons for deliberately failing to mention the whole Jerusalem fiasco. Duh.
We cannot form historical judgments based on happy fantasies that James and Paul got everything all worked out nicely. By all accounts, they didn't. Acts 15 never says one word about Paul's feelings about Jerusalem's decision, so it gives us no reason to expect Paul was or was not a fan of their letter. The church in Antioch took it as an encouragement. Of course they did. Circumcision was officially out of contention and the church at Antioch was 16 years old, mature enough to know it didn't have to obey Jerusalem's three laws! But Galatia was not yet so wise.
We can tell that Paul never mentioned Jerusalem's letter to Corinth because it was not until after Peter's visit that the Corinthians developed all kinds of controversy and questions about food, idols and 'es-ee-ex'. The fact that Paul does not mention Jerusalem's letter does not mean it did not yet exist. It could just as easily mean that Paul did not like the letter. OR, Paul could have been keeping that letter in reserve.
How did the Galatians know who Titus was? The simplest solution is that Titus carried the letter (probably together with Luke). If Galatians 2 is about the Council, Titus was an eyewitness to everything Paul writes about it. Titus was also in Antioch when Paul rebuked Peter. Titus also could have been holding Jerusalem's letter, in which case Paul would have no need to mention it in his own writing.
Put yourself in Galatia's shoes. Paul & Barnabas told you things about Jerusalem. The local Jews (in 2 of 4 cities) told you things about Jerusalem. Then the Judaizers told you things about Jerusalem. How confused would you then be about "the mother church" right up until the day Paul's letter shows up? Now put yourself in Paul's shoes. If you knew the Galatians were that confused about who and what Jerusalem really is, would you really expect them to believe "Jerusalem" agrees with you - and not with the Judaizers - about circumcision? That's why Titus holding Jerusalem's letter in reserve makes good strategy. Still doubting, Galatians? Okay. So now look at this.
Finally, put yourself in the historian's shoes. If we assume Galatians was written after the Council, what would that tell us about Paul's feelings for Jerusalem? Nothing that would be at all inconsistent with the rest of Paul's letters or his next two trips to Judea.
I don't know who Richard Longenecker is, but if I assume he's not an idiot, should I not then assume he's desperately fudging things on purpose? Or blindly following others who once made the same arguments? I don't know. You tell me.
Who benefits if we present a view of Paul submitting to the will of the established "mother church"? Who benefits if we present a church that became at peace politically after theological conflicts (James/Galatians) got ironed out? Who benefits if we present the high assembly of elders and apostles as an effective end to all dispute and division? Who benefits? You know who, saints. You know who.
The Jerusalem above is our mother. Come out of the slave woman.