Perhaps you should read more carefully.
Matthew 24:14 states that "this gospel of the kingdom shall be preached in all the world."
The command known as the Great Commission is given in Matthew 28:19 - "Go ye therefore, and teach all nations."
Now, if Paul were teaching only the Gentiles - as per your personal interpretation of Galatians 2 - why did he preach in the synagogues?
If Cephas were teaching only the Jews, why did he preach to the Gentile Cornelius?
Was each individual apostle sent to preach to each and every nation - or were the apostles allowed to divide up the task among themselves?
Further, why is the letter of Peter addressed to the strangers of, among other places, Galatia and Cappadocia?
If there were no apostolic hierarchy, why did Paul need the fellowship of James, Peter and John?
And why choose Matthias, if there were no succession?
Your interpretation has many holes that one could drive a fleet of Mack trucks through.
And why 12, THEN Paul?