First, it was the last leaf of the set of plates Joseph translated in Harmony. If the small plates of Nephi were in that set of plates, Joseph would have translated the small plates before leaving Harmony.
Second, the title page refers only to abridgments and Moroni’s sealing comments. No mention whatsoever of the original writings of Nephi and his successors.
Third, Joseph said the title page was part of the “original Book of Mormon,” meaning the plates that Moroni deposited in the stone-and-cement box. We don’t have the “original” Book of Mormon, partly because we don’t have the 116 pages (Book of Lehi) and partly because we do have the small plates of Nephi.
The two most extensive discussions of the Title Page I know of are Daniel Ludlow’s article here, and Alan Miner’s analysis here. Although they quote and analyze the Title Page extensively, neither one seems to have thought about the 3 points I mentioned above.
Here’s how Joseph’s explanation appears in History, 1838-1856:
“I wish also to mention here, that the Title Page of the Book of Mormon is a literal translation, taken from the very last leaf, on the left hand side of the collection or book of plates, which contained the record which has been translated; and not by any means the language of the whole running same as all Hebrew writing in general; and that, said Title Page is not by any means a modern composition either of mine or of any other man’s who has lived or does live in this generation.
Therefore, in order to correct an error which generally exists concerning it, I give below that part of the Title Page of the English Version of the Book of Mormon, which is a genuine and literal translation of the Title Page of the Original Book of Mormon, as recorded on the plates.”
Ludlow stops the quotation at “generation” so he never considers what Joseph meant by “the Original Book of Mormon.” Miner never considers that either. As far as I can tell, no one has analyzed that phrase before, although I’m sure someone has.
Elder Steven Snow wrote a useful background titled “Joseph Smith in Harmony.” It was published in the Ensign in September 2015 in connection with the Priesthood Restoration site. Here’s the link:
It includes a good map and some useful illustrations.
Jack Welch wrote a useful article on how long it took Joseph Smith to translate the Book of Mormon. It was published in the Ensign in January 1988, here: https://www.lds.org/ensign/1988/01/i-have-a-question?lang=eng#footnote6-03222_000_017
Source: Letter VII