Hyrum Smith, the scribe here in 𝓟, initially wrote the singular iniquity in verse 30, then immediately corrected it to the plural by erasing the y, overwriting the erasure with an i, and writing es inline. The following plural they and the parallel use of iniquities later on in verse 31 support the plural iniquities in verse 30. Theoretically, they and the subsequent use of iniquities in verse 31 could be considered the source for editing iniquity to the plural, but evidence elsewhere in the text supports the plural iniquities. As discussed under 2 Nephi 24:21, the scribes frequently mixed up the number for iniquity, so Hyrum’s error here is not unusual. Furthermore, when directly conjoined with the plural sins, we always get the plural iniquities (ten times), never the singular iniquity (here I include Mosiah 29:30 in the list):
In the case of Alma 37:10, the 1830 typesetter accidentally replaced the plural sins with the singular sin (for discussion, see under that passage). There is also one occurrence of singular sin conjoined with singular iniquity: “but teach them an everlasting hatred against sin and iniquity” (Alma 37:22). The critical text will in each case follow the earliest textual sources in determining the number for the word iniquity.
There is considerable evidence (all in Mosiah 29) that Hyrum Smith frequently mixed up the number for nouns. Twice he initially wrote plurals instead of singulars: sons instead of son in verse 7 and men instead of man in verse 12. In verses 16 and 31, he initially wrote the singular king instead of the correct kings (see below under Mosiah 29:31 for discussion). And in verse 18, he wrote abomination instead of abominations (there Oliver Cowdery made the correction in 𝓟). So this example in verse 30 of initial iniquity instead of iniquities is one more example of this error tendency on Hyrum’s part.
Summary: Accept in Mosiah 29:30 Hyrum Smith’s immediate correction in 𝓟 of iniquity to iniquities; the plural most probably reflects the reading of the original manuscript since the text has either all plural conjuncts or all singular ones for the phrase “sin(s) and iniquity/iniquities”.