The reading “the day of Nephi” in both manuscripts seems strange but possible, yet the 1830 typesetter changed the singular to the plural days, probably under the influence of the following plural days in “the days of Moroni”. The second example argues that the first example read in the original text as “the days of Nephi” (although one could turn this argument around and say that the first example argues that “the days of Moroni” should be emended to “the day of Moroni”).
Excluding the two cases in Alma 50:23, there are 28 occurrences of “the days of X” in the Book of Mormon text but none of “the day of X” (here X stands for a personal name). There is also one more instance of “the days of X” in the text, namely, “the days of Christ” (in Ether 13:4). I do not include this example in the count since Christ is not really a personal name but is a title referring to the Messiah. The important point here is that the 1830 typesetter’s emendation of “the day of Nephi” to “the days of Nephi” in Alma 50:23 is consistent with usage throughout the rest of the text.
There is considerable manuscript evidence that Oliver Cowdery tended to incorrectly write day instead of the correct plural days. For a list of six cases (including one in 𝓞), see under 2 Nephi 25:8. One of those cases is especially pertinent here since it also involves an instance of “the days of X”; in fact, it also has the same preposition, since :
In other words, in 3 Nephi 2:5 we have precisely the same error as is being proposed here in Alma 50:23, namely, “since the day of X” as an error for the correct “since the days of X”. But in 3 Nephi 2:5, Oliver caught his error and virtually immediately corrected day to days. Moreover, for that part of the text the 1830 edition is a firsthand copy of 𝓞, and it reads in the plural (“the days of Mosiah”). We should also note here that there is one more instance in the text of “the days of X” that takes the preposition since, namely, in Helaman 8:19: “even since the days of Abraham”.
The King James Bible has examples of “the days of X” (85 of them) but none of “the day of X”. Here are two examples with the preposition since (as in Alma 50:23):
Ultimately, the manuscript reading “the day of Nephi” appears to be an error for “the days of Nephi”. Usage elsewhere in the text as well as in the King James Bible consistently supports the plural “the days of Nephi”. And Oliver Cowdery was prone to replace days with day. The critical text will therefore accept the emended reading in Alma 50:23, “the days of Nephi”.
Summary: Accept the 1830 emendation in Alma 50:23 that changed “the day of Nephi” to “the days of Nephi”; this change is consistent with all other usage in the text, including “the days of Moroni” later on in this sentence; there is also considerable manuscript evidence that Oliver Cowdery tended to miswrite days as day.