How should we take it when Paul writes, "I say (I not the Lord)"? He does that a couple of times in Corinthians. I would think it is just as authorative (from God), but still why did he write that?
The majority of reliable commentators and the teachers I sat under say that the phrase "I say, I not the Lord" means what Paul writes immediately thereafter belong to his own understanding (still under divine inspiration) vs. what Christ taught him personally and specifically. Put another way, Paul is writing on a subject which he believes is in complete accord with the teachings of Christ by way of application.
Do you think Paul (or the other authors) always knew they were writing Scripture?
Peter recognizes Paul's letters as Scripture in 2 Peter 2:13, and Paul cites Luke as Scripture in 1 Timothy 5:8. If they didn't know at the time, it would have been interesting to hear Paul and Luke's reactions!