In Alma 12:1–3, scribe 2 of 𝓟 initially wrote plural forms, lyings (twice) and deceivings (once). In the first case (“his lyings and deceivings” in verse 1), scribe 2 immediately corrected these two plurals to singulars by erasing the final s ’s. These two corrections seem to indicate that scribe 2 was correcting the text to agree with the reading of the original manuscript, although one could view these two corrections as instances of editing on scribe 2’s part. In the second case (“thy lyings” in verse 3), the plural was not corrected by scribe 2, but the 1830 typesetter removed the plural s. One could argue that in Alma 12:3 scribe 2 made the same mistake as he did in verse 1 but in this case did not catch his error. On the other hand, the 1830 typesetter may have decided to make these two verses read consistently (namely, with the singular lying). In addition, the nearby singular craftiness in verse 3 might have motivated him to adopt the singular lying in that verse.
Usage elsewhere in the text shows that there is variation in number for the nouns lying(s) and deceiving(s). As far as the conjoined usage of these two nouns is concerned, we get plural forms in all cases except one (marked below with an arrow):
The last example is the only one where we get the singular lying when combined with deceiving(s), but this example of lying appears to be a gerundive noun, like the preceding robbing. One could argue that the original text here read lyings but that under the influence of the preceding robbing, lyings was mistakenly written as lying in 𝓞. For this part of the text, both 𝓟 and the 1830 edition are firsthand copies of 𝓞; thus the odds are quite high that for Mormon 8:31 the original manuscript read lying.
In the Book of Mormon text the noun deceiving(s) is always conjoined with lying(s), as in Alma 12:1. But there are instances where the noun lying(s) occurs without deceivings, as in Alma 12:3. For these cases, the plural is normal; but there is at least one instance with the singular lying in the earliest text (marked below with an arrow):
Of course, the singular lying in Alma 30:47 should, as in Mormon 8:31, be interpreted as a gerundive noun.
The last case listed above (in 4 Nephi 1:16) is complicated since in 4 Nephi both 𝓟 and the 1830 edition are firsthand copies of 𝓞. Scribe 2 of 𝓟 initially wrote the plural lyings (spelled as lyeings); then he immediately corrected the number to the singular by erasing the plural s. It appears that he may have accidentally written the plural lyings because of the surrounding plural nouns (“no envyings nor strifes nor tumults nor whoredoms ... nor murders”). There would have been no motivation for him to correct lyings to the singular unless 𝓞 read in the singular. Of course, this means that the 1830 typesetter also made the change to the plural lyings under the influence of the surrounding plural nouns. This example from 4 Nephi also argues that the changes in 𝓟 of lyings and deceivings to singulars in Alma 12:1 represent one other attempt by scribe 2 to get the number written down correctly in 𝓟. On the other hand, the 1830 change to the singular lying in Alma 12:3 could have been made, as already noted, under the influence of the conjoined singular form craftiness or the preceding singular lying in verse 1. Here at the beginning of Alma 12, the critical text will follow the earliest textual sources, accepting the singular forms lying and deceiving in verse 1 but the plural lyings in verse 3. For further discussion regarding the number for lying(s) in 4 Nephi 1:16, see under that passage.
Summary: Accept in Alma 12:1 the singular “lying and deceiving”, the immediately corrected reading in 𝓟; restore in Alma 12:3 the plural lyings in “thy lyings and craftiness”, the reading in 𝓟.