Even though Mormon has drawn parallels to the mission of the sons of Mosiah and the sons of Helaman, there is a major difference. The Lamanites have become sufficiently righteous that they send missionaries to the Nephites! This is an important development for two reasons. First, it shows that the Lamanites have truly become more righteous than the Nephites. Second, it sets the stage for the appearance of Samuel the Lamanite later in the book of Helaman. Mormon sets his readers up for that later inclusion by noting that he wasn’t the only righteous one, and that there was sufficient righteousness among the converted Lamanites that they would be able to send a prophet to go among the Nephites.
Verse 6 has Mormon continuing his emphasis on the lands northward. There are righteous Lamanites who go north, and Nephi and Lehi also go north. Mormon will briefly tell of that mission later.