Could the Writers of Scripture Have Been Influenced by Their Fallen Nature?

Questions about whether or not it’s reasonable to worry that some of our current doctrines were influenced by the fallen nature of the apostles, and how to defend sola Scriptura to Roman Catholics who argue that oral tradition carries as much weight as Scripture.  

• How much of the apostles’ fallen nature influenced their choice of words and how they communicated? Is it reasonable to worry that some of our current doctrines came from their just getting annoyed or impatient with someone and wording something too harshly?
• How do I defend sola Scriptura to my Roman Catholic family members in light of passages like 1 Thessalonians 2:13 and 2 Thessalonians 2:15, which seem to imply that oral tradition carries as much weight as Scripture?