There are strong reasons to believe both Peter and James existed. First, noncanonical gospels mention these men with great respect. They are key figures in every strand of Christianity. We only know Western Christianity. There is Middle East, Eastern European. North African, and Indian to name only a few.
I've read NT scholars bash the view that Jesus never existed. James, then Peter, were too important to not exist. St. Thomas gains a lot of credibility in India.
I know James and Peter never knocked on Paul's door with a surprise spy visit. Representatives from James and Peter, the Jerusalem contingent showed up and the people who learned Pauline Christianity from Paul were shocked by the likely eyewitness accounts of eye witnesses to Jesus the Human part. What is a light on the road to Damascus that no one else saw or voices that no one else heard compared to daily interaction with JC Superstar. People here keep thinking that the Middle East was full of country bumpkins. Travel was not as easy as today. Nevertheless, people did business in many cities on a routine basis. James and Peter may not have ordered the visits. A business person or tradesperson may have been in town on business and ran into a local Christian.
Latin was a common language. Huge segments of Europe spoke French for centuries. The current Latin/French is now English. It is not English because America is a wonderful country or b/c of our military clout. It is American because our most successful export is Hollywood films and TV shows. People are human. If you go to market or draw water, you learn things.
People in ancient times were sophisticated and very intelligent. All we have is some technology.
These facts are in Paul's letters. Perhaps not expressly but a long time before the Nag Hammadi and other finds, every scholar knew Paul was challenged by the Jerusalem brand and Gnostic brand. Paul is not writing theoretically. The people he cares about no longer believe his version. He writes to win them back. Sometimes he is too desparate for my taste. He almost grovels.
Confident people don't ever argue. People with power don't need to write on and on. He was emotionally involved. I don't know how to articulate it. I've read a lot of Paul and should stop coming here and read more Paul. The WT Paul never existed. If you read Paul's letter in one sitting, it is amazing. I do not read commentaries first. The text speaks for itself is the first step. Only after I sit and ponder the actual text, do I read scholarly material. No other denomination or scholar will ever do to me what the Witnesses did.