There are two textual problems here in Mormon 2:12. The first deals with whether lamentation(s) should be in the singular or plural. The printer’s manuscript has the plural (miswritten as lemantations), while the 1830 edition has the singular lamentation. Interestingly, the 1874 RLDS edition reintroduced the plural form, and all subsequent RLDS editions have followed this reading. (This later change shows that the plural is expected and can be independently introduced into the text; thus one could argue that the plural reading in 𝓟 is secondary.) On the other hand, the text of the early editions as well as the LDS text has maintained the 1830 reading, the singular lamentation. The question here is whether the word was in the singular or plural in the original manuscript.
Usage elsewhere in the text shows that in conjoined noun phrases, we always get number agreement between lamentation(s) and the other conjoined noun(s). First, there are four other cases where the singular lamentation is conjoined with the gerund mourning:
We get the same singular lamentation when it is conjoined with the nouns sorrow and howling:
The plural lamentations occurs only once in a conjoining of nouns, but in that instance the other nouns are also in the plural:
The plural is found in two other cases. In the first one, the plural lamentations occurs alone, without any conjoined noun:
The other case involves the conjoining of clauses (with ellipsis of the verb phrase, “was turned”); in this case the two nouns are not directly conjoined and occur at some distance from one another:
So when nouns are directly conjoined, lamentation(s) always agrees with the number of the conjoined noun(s); excluding the example in Mormon 2:12, there are seven instances of conjoined singulars and one of conjoined plurals (Ether 15:16), all listed above.
Here in Mormon 2:12, we have a conjoining of three nouns, each preceded by the determiner their: “their lamentation(s) and their mourning and their sorrowing”, which forms a noun phrase that acts as the direct object in the sentence. Thus Mormon 2:12 is most like the seven cases where we have the singular noun lamentation directly conjoined with another singular noun, such as mourning, sorrow, or howling. Thus internal evidence argues that Mormon 2:12 should follow the 1830 reading, with the assumption that scribe 2 of 𝓟 accidentally added the plural s. To be sure, scribe 2 of 𝓟 had a tendency to accidentally add the plural s. Elsewhere in the text there are at least 28 of these errors on his part: 21 are initial errors corrected by scribe 2 himself; 5 are errors corrected by Oliver Cowdery when he proofed 𝓟 against 𝓞; and 2 are in King James quotes where 𝓟 reads in the plural but the 1830 edition and the King James Bible have the correct singular. The critical text will therefore accept the 1830 reading in Mormon 2:12, the singular lamentation. (For another example of a word where its grammatical number always agrees with the nearest noun conjunct, see the discussion under 1 Nephi 19:11 regarding tempest.)
The second problem in this passage involves the word sorrowing. The earliest textual sources have the gerund sorrowing, which is matched by the preceding gerund mourning. The 1837 edition introduced sorrow in place of sorrowing. The 1908 RLDS edition restored the original sorrowing to the RLDS text, but the LDS text has maintained the 1837 sorrow. Elsewhere in the text, the Book of Mormon always uses sorrow rather than sorrowing in conjunction with other nouns, such as mourning, lamentation, grief, afflictions, pangs, troubles, care, and pain. In particular, there are the two instances of “great sorrow and lamentation” in Mosiah 9:19 and Helaman 6:33 (cited just above in the discussion regarding the number for lamentation).
Nonetheless, sorrowing does occur in the text—in fact, this gerund form is found twice in the very next verse after Mormon 2:12:
It seems quite clear in verse 13 that the word sorrowing is referring to the sorrowing just mentioned in verse 12. Thus the gerund sorrowing is fully supported in verse 12, both by the earliest textual sources and by nearby usage.
Summary: Maintain in Mormon 2:12 the singular lamentation (the 1830 reading) since we otherwise get the singular when lamentation(s) is directly conjoined with a singular noun (such as mourning); also restore the gerund sorrowing later on in the verse since the earliest textual sources have the gerund form; in addition, the following verse uses sorrowing, not sorrow.