The original text here clearly shows a subject complement in the singular (“we are a descendant of Joseph”). The 1852 LDS edition was the first one to emend this to the expected plural reading (“we are descendants of Joseph”). However, in the second printing of that edition, the original reading was restored (probably by reference to the 1840 edition). Similarly, the first two RLDS editions had the plural reading, but then the 1908 RLDS edition restored the original singular reading (undoubtedly because of the singular reading in 𝓟). Finally, the 1981 LDS edition edited the text to the plural.
There are three other passages in the text where we get this kind of singular subject complement:
In two of these cases, Joseph Smith edited “a descendant” to the plural, so that the 1837 and all subsequent editions have “descendants” rather than “a descendant”. In the remaining case, the change to the plural was first made in the 1852 LDS edition and has been retained in all subsequent LDS editions. The 1858 Wright edition and the first two RLDS editions also have the plural, but as in 1 Nephi 6:2 the 1908 RLDS edition reverted to the singular “a descendant” (based on the reading in 𝓟). However, in the two cases where Joseph edited the text in 𝓟, the 1908 edition retains the plural.
Elsewhere there are four cases where the earliest textual sources have the expected plural “descendants” as the subject complement:
Thus in the original text either the singular “a descendant” or the plural “descendants” is possible. Of course, when the subject is actually singular, we have only “a descendant”, as in 1 Nephi 5:14 (“he was a descendant of Joseph”) and 24 other places in the text.
It seems quite clear that the four unusual cases of the singular are fully intended. There is no attempt or even tendency prior to Joseph Smith’s editing for the 1837 edition to remove this singular subject complement. Nor is there any question here of being confused by a nearby singular subject and verb form. In each of the four cases the be verb is in the plural and agrees with a plural subject or antecedent (“we are”, “they are”, “these were”, and “which are”). The intentionality of the singular “a descendant” is obvious.
For each of these four cases, one could interpret the plural subject as semantically distributed rather than as a unitary collective plural. In other words, the plural subject could be interpreted as a group of distinct individuals, each of which is a descendant of X. Thus 1 Nephi 6:2 could be thought of as meaning something like ‘we are—each of us—a descendant of Joseph’. Another possibility, suggested by David Calabro (personal communication), is that the word descendant could be acting as a bare adjective without the expected noun but understood as referring to people. Thus the noun phrase “a descendant of Joseph” could be interpreted as ‘a descendant people of Joseph’—that is, a group of people descending from Joseph.
Summary: Restore the unexpected but fully intended “a descendant” that occurs with plural subjects and verbs in 1 Nephi 6:2, 2 Nephi 30:4, Alma 56:3, and 3 Nephi 10:4.