Oh? Just because some were prophets does not mean teachers cannot also be inspired. Of course prophesy requires inspiration of God,. Only He knows the future. All bible prophesy is right from God, of course. All the lessons from all the stories about people are also from God and are for our admonition and benefit to learn from! They are also inspired. The Scripture was inspired. God said greater works than what were seen will we also do. So that involves inspiration also. The gifts and calling of God involve inspiration also! The inspiration and moving of God has not stopped and is not limited to those who He inspired to get the record to us in written form. God breathes a lot. He breathed on Adam.when the Bible says, "All Scripture in God-breathed" (2 Timothy 3:16); and "knowing this first, that no prophecy of the Scripture came into being of its own private interpretation. For prophecy was not borne at any time by the will of man, but holy men of God spoke being borne along by the Holy Spirit" (2 Peter 1:20-21), it means that God the Holy Spirit DICTATED His Words to the Writers of the 66 Books of the Holy Bible, and they simply wrote what The Spirit told them to. This is ONE-OFF, and never to be repeated, which makes the Bible UNIQUE as the Only Word of Almighty God! There is not a single verse in the Bible that says any translation can be "Inspired"!
Relating this to the topic, I see no problem in Mark having written the ending for his inspired book later in life. That would not make it any less or more inspired.