This verse switches from the first person to the third person, perhaps because Mormon stopped directly quoting from Helaman’s letter and decided to summarize a longer passage. The last time this narrative specifically used the first person is in verse 50 (“and had I not returned with my two thousand / they would have obtained their purpose”). The narrative specifically returns to the first person in verse 54 (“and now it came to pass that we the people of Nephi—the people of Antipus and I with my two thousand—did surround the Lamanites”). All of verse 51, the first part of verse 52, and all of verse 53 could belong to the first-person narrative of the rest of the letter or to the third-person abridgment that definitely appears for most of verse 52:
Interestingly, when Oliver Cowdery copied verse 52 into the printer’s manuscript, he started to write I before Helaman at the beginning of the when-clause (“when I Helaman came upon their rear”), but before writing Helaman, Oliver tried to correct the I by overwriting it with an H (the first letter of Helaman). But the result was unclear, so he crossed out the I /H and wrote Helaman immediately following inline. Oliver expected the first-person narrative to continue, but obviously the original manuscript read in the third person here since Oliver intentionally corrected to the more difficult reading where the whole sentence is in the third person. Although there is a rather long lacuna in the original manuscript right after the when (so we cannot be sure whether the I was in 𝓞 or, for that matter, whether at the end of the line there was a his in “with his two thousand”), the text is extant at the end of the verse and reads “and turned upon Helaman”—that is, without any preceding me:
-it(t s) persueing them with great vigor when ( ) E HELAMAN CAME UPON THEIR REAR WITH HIS
two thousand & began to slay them excedingly in(so ) MUCH THAT THE WHOLE ARMY OF THE
Lamanites halted & turned upon Helaman now whe(n ) THE PEOPLE OF ANTIPUS SAW THAT THE
Of course, the printer’s manuscript clearly has the his and both instances of Helaman without any first person pronoun. The correction in 𝓟 of the I Helaman to Helaman is an immediate one; thus the occurrence of Helaman without any I undoubtedly reflects the difficult, but not impossible, reading of the original manuscript.
Stan Larson, on page 569 of his article “Conjectural Emendation and the Text of the Book of Mormon”, Brigham Young University Studies 18/4 (1978): 563–569, suggests that this sentence in verse 52 be emended to the first person; he also suggests that 𝓞 itself might have read I Helaman since this is what Oliver Cowdery initially wrote in 𝓟. But as we have seen, 𝓞 is extant for the last part of verse 52, and it reads “and turned upon Helaman” (without any me), so there is evidence that in 𝓞 this sentence read in the third person. Larson’s suggestion amounts to rewriting the text (which is acceptable, of course, as a revision, but not as a restoration of the original text).
The best solution here is to simply accept verse 52 (and maybe even the surrounding verses 51 and 53) as a third-person summary given by Mormon rather than as a direct quote from Helaman’s letter that somehow got messed up in the early transmission of the English-language text. There is no particular difficulty in reading this passage as it switches from first person to third, then back to first person (although the shift is clearly unexpected).
In a footnote to his discussion, Larson refers to another example of person shifting later on in the text:
In this case, we seem to have a case where an original we was twice misheard as ye, most likely prompted by the preceding use of ye in this verse: “and now when ye talk ye say”. There is good reason to accept the emended text in Helaman 13:25 (introduced into the LDS text in 1905 and into the RLDS text in 1953). But in Alma 56:52, there is no real possibility for simultaneously mishearing I Helaman as Helaman, my as his, and me Helaman (or simply me) as Helaman.
Summary: Accept in Alma 56:52 the original third-person usage in this verse; it is fully supported in 𝓟 and partially in 𝓞; the third person here seems to represent Mormon’s decision to briefly summarize rather than directly quote this part of Helaman’s letter.