This passage involves a manuscript change of who to that. Here Oliver Cowdery initially wrote who in 𝓟, then somewhat later edited the who to that. The level of ink flow is slightly heavier, which suggests that Oliver could have made the change while proofing the printer’s manuscript against the original manuscript. Another possibility is that the correction was virtually immediate and that Oliver redipped his quill just before correcting the who to that.
According to the earliest textual sources, the original text had 12 examples of “of him who” and 12 of “of him that” (where that is a relative pronoun), thus showing that both expressions “of him who” and “of him that” are possible. (I exclude Mosiah 4:12 from these statistics.) Although seven instances of “of him that” are found in quotations from Isaiah, the remainder are found within the normal Book of Mormon text. Four of these nonbiblical instances of “of him that” involve no variation at all with respect to the relative pronoun:
One instance has involved a change from that to who, the opposite of the manuscript change here in Mosiah 4:12:
For additional examples of the occasional tendency to change that to who (but in the 1840 edition), see the discussion under Mosiah 4:7. Also see under which in volume 3 for a complete list of this particular change. The critical text will accept the corrected reading in 𝓟 for Mosiah 4:12 since the change itself does not seem to be motivated by any grammatical consideration.
Summary: Retain in Mosiah 4:12 Oliver Cowdery’s corrected reading in 𝓟, which replaced “of him who” with “of him that” (the probable reading of the original manuscript).