It is important that you understand two things..Paul's evangel/gospel/teachings are not his, they are God's, and secondly they were revealed to him direct from the  Spirit of God.  

Paul, when he is referring to morals, ethics and social interaction, states that it is him speaking, emphasizing that everything else was revealed to him by the Spirit of God.

The evangel of the "uncircumcision", as it is considered, or the gospel of Paul to the Gentiles to which  it is more often referred, evolved over time. No doubt God's Spirit realized that too much, too fast, would literally "blow" Paul's mind.  Remember, Paul evolved from Saul ardent believer of the Law and chief persecutor of the followers of Jesus, to Paul a slave to Jesus, ardent missionary for the risen Christ.

That is one hell of a conversion!

Paul himself spoke of this evolvment over time, when he said, "When I was a child, I spoke as a child..when I became a man, I set aside childish things".

In the beginning, only one thing, essentially, set him apart from the "Twelve", the "Circumcision" Apostles..his gospel was available to all, one need not asume the ways of the Jews. The others insisted on proselyting pagans.

Then, gradually he was told:                    

All will be saved.. Jew and "Greek", slave..free man, male and female..ALL!                     

All sins are JUSITIED, due to the supreme sacrifice of our Lord Jesus Christ!                     

Those who BELIEVE, are set apart and are a part of the body of CHRIST!                    

Believers' lot will be in  the HEAVENS!

I believe the last revelation occurred after Paul's ascent into the heavens. 

The prison Epistles, Ephesians, Colossians, Philippians is when the latter truths  were revealed, because he realized that his time on earth was limited.

He also believed that his followers would be able to accept and understand the ultimate TRUTHS, as I assume you do.."when I became a man, etc..paulnbob