The 1858 Wright edition accidentally introduced the plural transgressions, which was followed by the first two RLDS editions. The third RLDS edition (1908) restored the original singular transgression, probably because this was the reading in 𝓟. In the Book of Mormon we always have the singular transgression in the expression “to fall into transgression” (15 times, including Mosiah 15:13). For further discussion, see under Jarom 1:10.
Summary: Retain the singular transgression in Mosiah 15:13, which is consistent with usage elsewhere in the text.