Here in the original manuscript, Oliver Cowdery initially omitted the and between brethren and all those. Virtually immediately he supralinearly inserted an ampersand (there is no change in the level of ink flow for his correction). Thus the corrected text distinguishes between “his brethren” and “all those which had come up” as fellow missionaries. Without the and, one could interpret “all those which had come up with him” (or, as we shall see, “all those which had come up with them”) as an appositive to his brethren. As pointed out by David Calabro (personal communication), there is evidence in the text for the general term brethren modified nonrestrictively by an appositive of the form “all those ”:
Since the appositive construction is possible, there is no motivation for Oliver to have added the and here in Alma 24:5 except that Joseph Smith must have dictated an and. Thus the critical text will assume that Oliver initially omitted the conjunction and in this passage as he took down Joseph’s dictation.
A more substantive question here deals with the use of him in “all those which had come up with him”. As explained under 1 Nephi 10:18–19, there is evidence that Oliver Cowdery sometimes mixed up him and them in 𝓞 since both are typically pronounced in unstressed contexts as /ßm/. In other words, it is possible that the original text here in Alma 24:5 read “Ammon and his brethren and all those which had come up with them” and that Oliver mistakenly interpreted Joseph Smith’s dictated /ßm/ as him rather than as them. In fact, the use of his in the immediately preceding “Ammon and his brethren” would have facilitated this interpretation. (This emendation was first suggested in 1998 by Dale Caswell, one of my student researchers in the critical text project.)
One wonders if there is any particular reason to associate the other missionaries with Ammon alone rather than with all four of the sons of king Mosiah. Support for the them is found earlier in the text; there the narrative consistently refers to these missionary companions as the companions of all four sons, not Ammon alone:
Of course, in all of these cases, the sons are collectively referred to as the “sons of Mosiah”, not conjunctively as “Ammon and his brethren”. On the other hand, when we have a full conjunctive listing of all the sons of king Mosiah, we always get the plural possessive pronoun their, never his, in referring to the missionary companions:
Of course, in these three passages the names of all four sons are listed, not just one of them; thus their, not his, is expected. (As discussed under Alma 22:35, the their in that instance may actually refer to just Aaron, Omner, and Himni.)
Here in Alma 24:5, it seems rather odd to refer to the missionary companions as coming up “with Ammon” rather than with “Ammon and his brethren”, especially since Ammon worked alone as a missionary. Alma 24:5 is the only passage that refers to the other missionary brethren as being Ammon’s brethren rather than the brethren of all the sons of king Mosiah. Thus all of the relevant passages elsewhere in the text support emending Alma 24:5 to read “and all those which had come up with them”. The critical text will accept the emendation of him to them in this passage.
Summary: Emend Alma 24:5 to read “and all those which had come up with them” since elsewhere the text consistently refers to the missionary companions as the companions of all the sons of king Mosiah; the original them, probably pronounced as /ßm/ by Joseph Smith, was apparently misinterpreted as him by Oliver Cowdery as he took down Joseph’s dictation; Oliver was probably influenced by the immediately preceding his in “Ammon and his brethren”.