Here the printer’s manuscript has the prepositional a before the present-participial prophesying, but the 1830 edition lacks it. One possibility is that Oliver Cowdery, the scribe for 𝓟, may have accidentally added the a. Note, for instance, that none of the three other present-participial conjuncts in this sentence have the prepositional a. When we look at the earliest text, we find that most instances of the prepositional a occur in nonconjunctive contexts, as originally in 1 Nephi 8:28: “they were ashamed because of those that were a scoffing at them”. But there are two other passages where the prepositional a occurs in a conjunctive context; both refer to mourning and are very similar in their structure:
In the first passage, the earliest text (here the printer’s manuscript) has two instances of a mourning, but the 1830 compositor removed both a ’s when he set the type for that passage. In the second passage, as explained under Alma 28:5, the original text apparently had the prepositional a for the second and third instances of mourning but not for the first one. Oliver Cowdery, in his own editing in 𝓞, removed the second one (perhaps because the first one lacked the a) but kept it in the third case when he copied the text from 𝓞 into 𝓟 (although initially he omitted it there). Once more, in this third case, the 1830 compositor removed the a from before mourning when he set the type.
From the example in Alma 28:4–5 we may conclude that in conjunctive contexts the prepositional a can occur for some of the conjuncts but be lacking for others. Thus the reading in 𝓟 for Helaman 16:4 is possible. (There is one other case where Oliver appears to have editorially removed an original prepositional a, namely, in Alma 55:8; for discussion, see under that passage.)
Excluding the case here in Helaman 16:4, there are a total of 25 cases where the copytext for the 1830 compositor had the prepositional a; and in ten of those cases, the compositor omitted the a, including a preceding case here in the book of Helaman:
On the other hand, there is not one example where Oliver Cowdery added a prepositional a when he copied from 𝓞 into 𝓟. Wherever 𝓟 has the prepositional a and 𝓞 is extant, then 𝓞 has the a (there are seven examples). There is one case in 𝓞 where Oliver accidentally inserted the prepositional a, but there he immediately caught his error and erased the a:
Thus here in Helaman 16:4 the odds are considerably greater that the 1830 compositor deleted the a than Oliver Cowdery added it. The critical text will therefore restore the reading in 𝓟 with its mixture in usage: “Nephi was baptizing and a prophesying and preaching and crying repentance unto the people”. For a complete discussion of this archaic aspect of the text, see under prepositional a in volume 3.
Summary: Restore in Helaman 16:4 the prepositional a, the reading in 𝓟: “Nephi was … a prophesying”; the 1830 compositor frequently deleted the prepositional a when he set the type, but evidence from extant portions of 𝓞 show that Oliver Cowdery never added any prepositional a on his own when he copied from 𝓞 into 𝓟.