Here Oliver Cowdery initially wrote the plural iniquities in 𝓟, perhaps because of the following plural in the conjoined “their transgressions”. He immediately corrected the plural iniquities by erasing the final s and overwriting the ie with y. The original manuscript undoubtedly read iniquity. See under 2 Nephi 24:21 for a list of examples where the scribes initially mixed up the number for the noun iniquity.
Summary: Maintain the singular iniquity in Mosiah 15:9, the immediately corrected reading in 𝓟.