For one, Paul and Barnabus spoke after Peter did.
Yes they did and they no longer spoke about circumcision. The matter was settled. We also have the example in Acts 10 where an angel instructed Cornelius to send for Peter. Peter went to him and became the first apostle to receive gentiles into the church and the first to baptize them.
"And he commanded them to be baptized in the name of Jesus Christ. Then they asked him to remain for some days. "(Acts10:48)
What Peter speaks about in Acts 15 is what he had already done in Acts 10. Peter not only made the decision, he had already implemented it.
A careful reading of II Peter 1:16-19 would indicate that Peter (your 1st Pope)believed Scripture to be more reliable than God's audible voice. In verse 19 he refers to scripture as a "more sure word".
A more careful reading of 2 Peter will show that Peter did not indicate that scripture was more reliable than God’s word. What Peter was referring to was God’s voice validated the prophetic word. That’s why he said “And we have the prophetic word made more sure. “(2Pet 1:19)