Oliver Cowdery, in proofing 𝓟 against 𝓞, added the plural s to Hyrum Smith’s original singular abomination. Since the parallel language that follows uses the plural (“and also the wickedness and abominations of his people”), one could argue that Oliver consciously emended the text here rather than correcting to 𝓞. The earliest text has 40 invariant instances of the plural abominations conjoined with wickedness. But there are three instances where the earliest text clearly has the singular abomination conjoined with wickedness:
There is one other case with conjoined wickedness where there is some early variation in number for abomination:
This example may be in that part of the text for which the 1830 edition was set from the original manuscript rather than from the printer’s manuscript. Internal evidence suggests that in Helaman 13:17 the original text read in the plural (“because of their wickedness and their abominations”). For discussion, see under that passage.
There is one place in the manuscripts where we have clear evidence of Oliver Cowdery struggling with determining the number for abomination:
Here Oliver initially wrote abominations, then almost immediately corrected it to abomination by crossing out the plural s. As discussed under Jacob 2:28, the textual evidence argues that the singular is the original reading. But other than this example, Oliver seems to show no inclination to emend the number for abomination. Thus his correction of abomination to abominations here in Mosiah 29:18 is probably the result of his proofing 𝓟 against 𝓞.
Summary: Maintain in Mosiah 29:18 Oliver Cowdery’s correction of abomination to abominations since generally Oliver was not inclined to emend the number of abomination.