Here we have in 𝓟 an initial slip by Hyrum Smith, who virtually immediately corrected the singular king to the plural by inserting the plural s inline. Note the occurrence of the plural kings in the following clause. A similar instance of Hyrum omitting the plural s for kings is found earlier in this chapter:
The plural is required in verse 16. We will therefore follow the corrected reading in verse 31 as well: “the iniquities of their kings”.
Summary: Maintain in Mosiah 29:31 the plural kings (the almost immediately corrected reading in 𝓟).