Paul was the supreme example of the grace he was sent forth to proclaim. (1 Tim. 1:12-16).
But he was also accountable to God for everything he received and taught concerning his calling and message. This is why he vigorously defended his apostleship.
"For I speak to you Gentiles, inasmuch as I am the apostle of the Gentiles; I magnify mine office.
But I certify you, brethren, that the gospel which was preached of me is not after man.
For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ." Rom. 11:13 and Gal. 1:11,12.
He could not have been taught by the 12 because his gospel was taught him, not by Peter or the 11, but by the revelation of Jesus Christ.
If you read through his epistles, Paul first hustled himself to the 12 in order to be accepted and constantly complained that they didn’t accept him as equals to them. If Paul was so sure, why would he complain so much?