Can You Explain Verbal Plenary Inspiration When the New Testament Writers Quote the Septuagint?

Questions about inspiration when the New Testament writers quote the Septuagint rather than the original text, whether we could add books to the Bible if new letters from the apostles were found, and how to handle passages like John 7:53–8:11 that were added to the original biblical text.   Can you explain verbal plenary inspiration with regard to the New Testament writers quoting the Septuagint in places where it’s different from the original Hebrew text? Would it be inappropriate to add books to the Bible if new letters from the apostles were found? How should we handle passages like John 7:53–8:11 and Mark 16:9–20 that seem to have been added to the original biblical text?