Here in the original manuscript, Oliver Cowdery initially wrote “who he so dearly beloved”. Virtually immediately Oliver corrected the he by crossing it out and supralinearly inserting the correct they (there is no change in the level of ink ﬂow). Oliver was probably inﬂuenced by the preceding use of the singular Ammon and the his of the conjoined “and his brethren”, but the following conjoined prepositional phrase shows that the plural is correct: “and among those who had so dearly beloved them”.
The relative pronoun who occurs twice in this passage, although in the first instance the who serves as the direct object in the relative clause. Thus the who was edited to whom in the 1840 edition (presumably by Joseph Smith). But the original who was restored to the RLDS text in the 1908 RLDS edition since 𝓟 reads who (there is no correction to whom in 𝓟); in his editing for the 1837 edition, Joseph did not make the change to whom. The 1920 LDS edition made the grammatical correction to whom for the LDS text. The original who is obviously intended, given the parallelism between the two relative clauses:
among those who they so dearly beloved
and among those who had so dearly beloved them
For further discussion of who versus whom, see under pronouns in volume 3.
Summary: Maintain in Alma 27:4 the plural pronoun they, the corrected reading in 𝓞 (“among those who they so dearly beloved”); also restore who, the original form for the direct object relative pronoun in this passage.