In English we have two ways of expressing the subject for a gerundive verbal: the subject can take either the possessive form or the object form (as in “the cause of his becoming carnal” or “the cause of him becoming carnal”). The printer’s manuscript here reads “the cause of all mankinds becoming carnal”; the final s should probably be interpreted as the possessive s, which Oliver Cowdery typically wrote without the apostrophe in the printer’s manuscript.
Here in Mosiah 16:3, the 1830 typesetter replaced the possessive form mankind’s (written as mankinds in 𝓟) with the object form mankind. Other places in the text show that the Book of Mormon characteristically uses the possessive in this context, although all the examples (or potential examples) involve a possessive pronoun rather than a nonpronominal noun phrase like “all mankind”:
One of these pronominal cases could be interpreted as an object form—namely, 3 Nephi 7:8: “her wallowing in the mire”; but this is only because her is both the possessive and object form of the pronoun she. Given the possessive pronominal forms in all the other cases, we should interpret 3 Nephi 7:8 as a case of the possessive pronoun, not the object pronoun.
In addition, there are two gerundive instances with possessive pronouns that have been created by later editing of the text:
Besides the one case of “all mankind’s” in Mosiah 16:3, there are two other cases where a nonpronominal noun phrase could be interpreted as a possessive:
In the first case (Mosiah 9:4), the noun phrase “many days” has been treated, beginning with the 1830 edition, as a possessive (by explicitly providing an apostrophe). This interpretation appears to be unnatural given that the noun phrase “many days” should probably be treated as the object of the preposition after or as an adverbial. Typically, such noun phrases are not interpreted as possessives in standard English, as in the sentence “After many years living in poverty, Jim struck it rich.” (For further discussion of this first case, see under Mosiah 9:4.) In the second case listed above (Moroni 4:1), there has been no addition of apostrophes; that is, no printed edition has set the text as “the manner of their elders’ and priests’ administering the flesh and blood” (or “the manner of their elders and priests’ administering the flesh and blood” if one interprets the conjunctive “elders and priests” as combinatory rather than segregatory).
The occurrence of the noun phrase “all mankind” as a possessive in Mosiah 16:3 appears to be fully intended, despite its unique usage within the text; the possessive reading is clearly possible. For this reason, the critical text will accept the reading of the printer’s manuscript, with the understanding that the s of mankinds is the possessive ending.
Summary: Interpret all mankinds (the reading in 𝓟 for Mosiah 16:3) as a possessive form of the noun phrase all mankind, thus restoring the original text (“the cause of all mankind’s becoming carnal”); this reading is supported by numerous cases where the subject for a gerund takes the possessive form (although all these other examples are possessive pronouns).