Textual: Mormon provides some more information about the way in which he structured his writing, but unfortunately complicates the picture as well. Part of the problem is in the selection of verb tenses.
Mormon will "finish [his] record upon" the small plates. He "shall take" the remainder of his record from the plates of Nephi. In connection with that larger collective name of "plates of Nephi," he indicates that he "cannot write the hundredth part of the things of my people."
The problem is the juxtaposition of "finishing" on the small plates, and then continuing to write the bulk of our current Book of Mormon starting at Mosiah. If Mormon is near death when he is writing this interlude known as Words of Mormon (as suggested by verse 1) he does not have time left to abridge the records from Benjamin to his own time given his proximate demise.
Daniel H. Ludlow summarizes:
"In verse 5, Mormon mentions that he is going to finish his record upon "these things" and that he will take the remainder of his record "from the plates of Nephi." Several questions have been raised concerning this brief verse by Mormon: (1) First of all, to what is he referring when he states he is going to finish his record? (2) To what plates is he referring when he says he will finish his record upon these things? (3) To what section of his writings is he referring when he talks of the remainder of his record?
Most Book of Mormon scholars have assumed that when Mormon refers to finishing "his record" he had in mind the rest of his writings in the small section entitled The Words of Mormon. Most scholars also assume that "these things" refer to the small plates of Nephi. If this interpretation is correct, then the section entitled The Words of Mormon was written at the end of the small plates of Nephi. According to these scholars, the fact that Amaleki says the small plates of Nephi are already full (Omni 30) does not necessarily rule out the possibility of adding the brief notes that make up The Words of Mormon.
Mormon's reference to the "remainder" of his record is a little more confusing. Some scholars believe that here Mormon is referring to the rest of his writings in The Words of Mormon, the ideas of which he obtained from the large plates of Nephi. Other scholars, however, believe that Mormon is referring to that portion of his abridgment from the large plates of Nephi which he has not yet written on his plates of Mormon. Unfortunately, the pronoun reference in verse 5 does not make it possible to determine Mormon's meaning exactly." Ludlow, Daniel H. A Companion To Your Study Of The Book Of Mormon. Deseret Book 1976, pp. 171-172.)
Mormon's record is very clearly the abridgment he is creating from the plates of Nephi. The most interesting suggestion here is that the Words of Mormon appear physically on the small plates. That explains the "upon" in a very literal way. However, it is not a necessary addition, as Mormon is equally as clearly adding these plates just before giving the set of plates to Moroni, and this the act of inclusion is equally a "finishing upon" as writing on the physical plates would be. The suggestion that Mormon still have the Mosiah to 4 Nephi section of the plates left to write contradicts the intent of verse 1 and does not appear to be a viable solution (although Eldin Ricks appears to hold this position, see Ricks, Eldin, "The Small Plates of Nephi and the Words of Mormon." The Book of Mormon: Jacob Through Words of Mormon, To Learn With Joy. BYU,1990, p. 216).