We first note here that this passage contains a sentence fragment: the noun phrase the army, followed by a series of relative clauses, is never completed. Instead, after describing the position of this other army and who its leader was, the text starts over, so to speak, with an independent clause (“and he led his army forth and encircled the Lamanites about on the east in their rear”). The preceding relative clauses have never been altered to eliminate the fragment. This construction appears to be fully intended and will be retained in the critical text.
A more substantive question in this passage is whether who is the appropriate relative pronoun to refer to army (“the army … who was led by a man whose name was Lehi”). Notice that the preceding relative clause has which, not who: “the army which was concealed on the south of the hill”. Before writing the who in 𝓞, Oliver Cowdery initially wrote whose, an error in anticipation of the following whose (“by a man whose name was Lehi”). Oliver immediately caught this error and crossed out the whose, then continued by writing inline who. It is possible that this who was an error for which; having just written whose, Oliver could have just crossed out the se of whose to get who. Instead, his intention could have been to write which, so he crossed out the entire whose; but because he had just written whose, he ended up writing who.
Except for this one case in Alma 43:35, the earliest text always uses the relative pronoun which, never who, to refer to an army or to armies (14 times):
In three of these cases, Joseph Smith emended the which to who (although Joseph changed his mind in the last of the three cases):
In the two emendations that were kept, there are plural elements (set above in bold) that indicate that the individuals in the army are being considered, and thus the grammatical change to who is acceptable. But here in Alma 43:35 there are no plural elements to justify the use of who. (For further discussion of the possibility of treating an army as a unit or as a group of individuals, see under Mosiah 23:25.)
Of particular importance here is the striking similarity between the language of Alma 43:35 and Alma 50:35; in both cases, army is followed by two asyndetically conjoined relative clauses and, most importantly, the two instances of the second relative clause are perfectly parallel, providing we emend who to which in Alma 43:35:
Alma 43:35 (who emended to which) Alma 50:35
the army the army
which was concealed on the south of the hill which was sent by Moroni
which was led by a man which was led by a man
whose name was Lehi whose name was Teancum
Since the who in 𝓞 for Alma 43:35 is readily explainable as a repeated error in anticipation of the following whose, the critical text will accept Joseph Smith’s emendation of who to which in this passage as the reading of the original text.
Summary: Follow Joseph Smith’s emendation of who to which in Alma 43:35 (“the army … which was led by a man whose name was Lehi”), which precisely parallels the construction in Alma 50:35: “the army ... which was led by a man whose name was Teancum”; the who in 𝓞 for Alma 43:35 can be explained as the result of twice anticipating the following whose; the two relative clauses that modify army in this passage form a sentence fragment that has never been emended (and will be retained in the critical text).