Dirk, wow, I'm no theologian. My dad was, and I learnt much from him, but I'm not one to speak with any great authority on the early church fathers. I do agree with you that they did have a tendency to wander off into their own personal specifics that divided into various sectarian branches, from what I remember of what my dad taught me.
Paul? Well, he's a thorn in the craws of many, many people. Those who are part of the feministic theology branch hate him with a passion as being a "woman hater," which I don't see as being possible when reading ALL that he said about various women in his letters.
Your first question is a good one that I did ask my dad about many years ago, so I'll try to just give you the gist of what he shared with me. Paul, then he launched out into territory not specifically addressed by the rest of the Bible, would say clearly, "I say, not the Lord...." Just my paraphrase. He had the integrity to tell his readers that he was making a statement not directly told him by the Lord.
Now, you personally have already (or will) set the bar of your acceptance of Paul's words, but for me, I see his launching out into judgements not directly spoken to him by the Lord are based on his assessment of the totality of God's word rather than developing some new doctrine seen nowhere else, and that doesn't mesh well with other areas of the Bible. If you have a specific verse, or verses in mind, then we might be able to address your question more closely to your concern.
I'm not sure exactly how you define pre-marital sex, but if it's based on post-modern, social dogma, then it is indeed the sin of fornication when it's done to "get some" before settling into a commitment for marriage. I have no regard for marriage certificates. They mean nothing. What matters to God is what's in the heart of the couple. If they are truly committed to one another until death, then their physical union is not sin in the eyes of God, because God defined marriage in Genesis 2 once and for all. Typing all my dad's messages for him in posts really drove that home to me. There are three elements that seal a marital relationship, and anything outside that is either fornication or adultery. My dad REALLY got some people's attention when he stated that many with those certificates from the government were, or are STILL living in the sins of fornication/adultery considering the divorce and remarriage rates in this country.
Once my dad helped me to see Paul as being the Lord's true servant, I began to see even more the glory of Christ Jesus and the greatness of His salvation that He has given to us. Paul's words, although sometimes harsh, but balanced with a level of love so far above that of even the church fathers, no. I'd say he was indeed on an apostolic level of service to the Lord. His work for the Lord speaks too loudly for us to deny how great the Lord is.
It is the Lord's magnificence that inspired to me create this remix that I finished last night:
https://soundcloud.com/https%3A%2F%2Fsoundcloud.com%2Ffourwindsangels%2Fmighty-salvation-remix