Redaction: This is Mormon’s comment, a signal that he is interjecting a detour to his original subject. Nevertheless, it is hard to understand precisely what the detour was, because Mormon returns to the subject of the detour. What was Mormon’s original intent?
If we were to remove verses 4–19, we would have a reasonably consistent narrative of the events moving directly from verse 3 to verse 20. It would appear that the insertion was triggered by verse 4’s introduction of the mark of the Amlicites. In verses 4–12 Mormon explains the mark and discusses the Lamanite curse from a Nephite perspective. In verse 13, he signals that his detour is over and that he is returning to the story of the Amlicites. Therefore, it seems that he has considered the discussion about the Lamanite curse to be the detour, for he immediately returns to the story of the Amlicite marking and how that fulfilled commandments.
From this information, we can reconstruct Mormon’s intent. In his narrative, Mormon pauses to make a specific moral point: that the Amlicites are wicked in their rebellion and that they are fulfilling a curse by marking their foreheads. Mormon frequently uses history to frame moral stories. His detour is the discussion of the Lamanites. While it is certainly related to what Mormon wants to say, he communicates that it is only a tangent to his main point: that the Amlicites fulfill prophecy and are therefore still Yahweh’s tool even though they have rebelled against Yahweh.