Here in 𝓟, Oliver Cowdery initially wrote “his Son had been appointed by the people in his stead”. Virtually immediately Oliver supplied the relative pronoun which (the supralinearly inserted which was written with no change in the level of ink flow). The relative pronoun seems necessary since otherwise we get a finite clause immediately followed by an unattached predicate. This kind of construction occurs nowhere else in the Book of Mormon text. Oliver’s inserted which does not seem to represent editing, but rather his attempt to get the reading of 𝓞 down correctly in 𝓟. For a list of other cases where the scribes omitted the relative pronoun which (usually momentarily), see under Alma 5:3. For the later editing of which to who in Helaman 6:15, see under which in volume 3.
Summary: Maintain the relative pronoun in Helaman 6:15: “his son which had been appointed by the people in his stead was also murdered”; Oliver Cowdery’s addition of the which here in 𝓟 was apparently the result of his making sure the reading in 𝓞 was correctly copied into 𝓟.