To fully understand the impact of these passages, one must be acquainted with how the Book of Mormon was translated by Joseph Smith. At what point in the translation process were the Small Plates of Nephi translated? At the end, the very end. After Joseph Smith completed the translation of the rest of the Book of Mormon, he found these plates (the Small Plates of Nephi) and translated them.
Put yourself in the position of Joseph Smith. The first 116 pages of the Book of Mormon were lost and Joseph was devastated that he allowed that to happen. For a period of time, his gift to translate was taken away from him. Finally, Joseph was forgiven, but he was instructed to not go back and retranslate the 116 pages that had been lost. He was to start translating where he had left off when the translation process had been disrupted—which was early in the book of Mosiah. That is where Joseph Smith resumed the translation process. So, think about this: the whole time Joseph Smith was translating from Mosiah to Moroni, he was probably thinking about all the effort that Mormon had put into abridging the book of Lehi in the first part of the record and regretting that those who would read the Book of Mormon in modern times would not have those words of Lehi.
Imagine the joy that Joseph Smith felt when he finally got to the end of the plates and he read Mormon’s words saying: "Hey, I am appending this record here. I do not know quite why, but the Spirit is whispering for me to do it."
We talk about God, His omniscience and all His knowledge. This very statement is made in Words of Mormon 1:6–7. God knew fourteen hundred years in advance that those 116 pages of manuscript would be lost. Maybe cutting his editorial teeth on that part of the record had served as a means for Mormon to perfect his editing style. Perhaps by the time he got to Mosiah, Mormon was doing really well with this task.
By having Nephi’s own words, Mormon knew that we would have the benefit of hearing Nephi’s voice at the beginning of the Small Plates, as what would have been a second witness to the words of Lehi. Nephi’s words are beautiful. As we read the record of Nephi, we become aware that this was not a one-time journal that he was keeping. Nephi was writing other records and was instructed by the Lord to rewrite things. What we have in 1 Nephi is his final revision of several records. The record we have is something that Nephi labored over, and he got things beautifully and wonderfully correct. I am grateful to him for that, and in the end, it is a blessing from the Lord that we have it this way.
What about Mormon himself? Are you grateful to Mormon for what he did? Here was a young, 16-year-old general who was called to lead his people. He was able to look back on a civilization that he was very proud of. He was a long inheritor of a great tradition which he knew was going down. He was well aware of the prophecies as well as the spirit of the time and yet, in spite of everything he could do, he did not throw in the towel. He gave it his best shot. Mormon tells us that the people living in his time would not listen, but that he was going to leave a record that would influence the world hundreds and thousands of years hence. What a courageous spirit it took for him to respond to challenges in that way.
Book of Mormon Central, "How Does the ‘Mosiah-First’ Translation Sequence Strengthen Faith? (Words of Mormon 1:5)," KnoWhy 503 (February 22, 2019).
"Besides making clear sense of the manuscript and historical data, this understanding of the translation sequence [Mosiah–Words of Mormon] also provides fascinating and somewhat unexpected evidence for the Book of Mormon’s authenticity. This is because many passages from Mormon’s abridgment of the Large Plates, as well as from the books of Mormon, Ether and Moroni, refer back to content from the Small Plates, even though the Small Plates hadn’t even been translated yet!"
Book of Mormon Central, "What Was on the Lost 116 Pages? (1 Nephi 9:5)," KnoWhy 452 (July 24, 2018).
"Whatever challenges or setbacks we may suffer in our personal lives, we can … trust that the Lord can compensate for our weaknesses and limitations. Sometimes, like Joseph Smith and Martin Harris, we may feel that ‘all is lost.’ Yet through the infinite power of Christ’s Atonement, all that seems lost can be ultimately and miraculously restored."