Here the printer’s manuscript has the plural form dissensions (𝓞 is not extant for the word). The plural form agrees with the preceding plural wars and contentions as well as with the following plural prophecies. The 1830 compositor set dissensions in the singular, but the 1837 edition restored the original plural. Elsewhere in the text, we always get the plural dissensions when conjoined nouns are in the plural (11 times), as in the following examples where dissensions is conjoined with both wars and contentions:
The critical text will follow the original plural reading, dissensions, in the preface to the book of Helaman.
Another question here in the Helaman preface is whether there might be a missing their in the conjunctive phrase “their wars and contentions and their dissensions”. 𝓞 is sufficiently extant to determine that there was no their before contentions. Nonetheless, there is evidence that repeated their ’s were sometimes omitted from the text, as in the following example involving this same phraseology:
On the other hand, determiners for conjuncts of semantically close nouns are sometimes not repeated, even when the following conjoined noun does repeat the determiner, as in the following example:
The two nouns wars and contentions are closely associated, which means that the determiner is frequently not repeated for the phrase “wars and contentions” (as here in the Helaman preface):
Note, in particular, that three of these examples specifically refer to an “account of the wars and contentions” without any repetition of the determiner (just like here in the Helaman preface). Thus there is no need to repeat the their in the Helaman preface, despite its being followed by “and their dissensions”.
Summary: Accept in the Helaman preface the plural dissensions as well as the nonrepetition of their in the phrase “their wars and contentions”; both readings are supported by the earliest textual sources as well as by usage elsewhere in the text.