He clearly wrote them as commentary on the existing Scriptures, which means that they can't simply be read as stand-alone pieces if you want to understand his real points.I cannot confirm that Paul knew that he was writing scripture, but just commentary? Is there something new in Paul's letters not found elsewhere in scriptures? If not, why are his letters even included in scriptures?
Somewhere along the line it had to be revealed that the letters were inspired, or we would not know even now.
Actually, there's very little of Sha'ul's letters that is distinctly new. He almost always draws on existing Scripture or the experiences of the early Church (e.g., the Holy Spirit coming to the Gentiles without circumcision) and logical inferences drawn from them to build his theology. There are exceptions, of course, such as the mystery proclaimed in 1 Cor. 15, that the living would be "resurrected" and transformed along with the dead at the Lord's Coming, but by and large he exposits rather than prophesies.
That being the case, my argument is simply that we also need to be intimately familiar with the source material that he is expositing from in order to correctly understand Sha'ul's writings.