Oliver Cowdery originally wrote “by Alma and Amulek and Zeezrom” in the printer’s manuscript; shortly afterwards he supralinearly inserted (with no change in the level of ink flow) the preposition by before Zeezrom—but not before Amulek, thus creating a mixed conjunctive structure (“by Alma and Amulek and by Zeezrom”). In the 1837 edition, the extra by was deleted, probably because of the perceived awkwardness of the mixed structure.
Nonetheless, the corrected text in 𝓟 is appropriate. We first note that Alma and Amulek worked together as a missionary pair (as exemplified by their successive discourses in Alma 9–13 and in Alma 32–34). Of course, when they taught in the city of Ammonihah, Zeezrom was not yet converted, so his discourses would have been later (such as during the missionary work among the Zoramites). Zeezrom is definitely not as closely associated with Alma and Amulek as they are with each other. Of course, Zeezrom is still associated with Alma since he is listed as one of the missionaries that accompanied Alma on the Zoramite mission (Alma 31:6, 32). Elsewhere the Book of Mormon text reflects an extra closeness between Alma and Amulek; whenever their names are conjoined in a prepositional phrase, the preposition is never repeated:
Thus this extra-close relationship between Alma and Amulek is shown in Helaman 5:41 by the lack of the preposition by before Amulek but not before Zeezrom. (For a general discussion concerning whether the preposition by should be repeated in conjunctive structures, see under Alma 2:38.)
Summary: Restore in Helaman 5:41 the preposition by in front of Zeezrom (“by Alma and Amulek and by Zeezrom”); the lack of the by in front of Amulek reinforces the special closeness between Alma and Amulek.