Oliver Cowdery’s correction in 𝓟 of iniquity to iniquities is just like the previous one of mine to my. The y of iniquity is crossed out and the correct ies is supralinearly inserted, with darker and heavier ink flow from a broader or duller quill.
Either the singular or plural will work here in 2 Nephi 4:17. In fact, if we consider all examples in the text of a possessive pronoun preceding iniquity or iniquities, it is very clear that either singular or plural is possible (even though the plural is almost twice as frequent):
| my iniquity | 0 | my iniquities | 1 |
| mine iniquity | 0 | mine iniquities | 1 |
| thine iniquity | 1 | thine iniquities | 4 |
| his iniquity | 2 | his iniquities | 4 |
| our iniquity | 2 | our iniquities | 6 |
| your iniquity | 5 | your iniquities | 16 |
| their iniquity | 25 | their iniquities | 30 |
In all, we have 35 occurrences of the singular and 62 of the plural. There was therefore no grammatical motivation in 2 Nephi 4:17 for Oliver Cowdery to have emended “mine iniquity” to “mine iniquities”. Most probably, his correction in 𝓟 was simply to make the text agree with the reading in 𝓞.
Summary: Maintain in 2 Nephi 4:17 the plural “mine iniquities”, the corrected reading in 𝓟.