Since the 1953 edition, the RLDS text has here read transgressions rather than the singular transgression. This difference is probably a typo since the 1953 edition otherwise set every instance of “fall into transgression” in the singular. In all, there are 15 instances in the original text of “to fall into transgression” but none of “to fall into transgressions”, as in these two other examples from king Benjamin:
For further discussion, see under Jarom 1:10 and 3 Nephi 6:5. The critical text will maintain the singular transgression in Mosiah 7:25.
Summary: Retain in Mosiah 7:25 the singular transgression in “if this people had not fallen into transgression”.