In the printer’s manuscript, Oliver Cowdery initially wrote “the one half of their lands”, probably because the preceding phrase reads “the one half of their property”. Oliver supralinearly inserted the universal quantifier all in 𝓟 and without any change in the level of ink flow, so the correction is virtually immediate and undoubtedly represents the reading of the original manuscript.
This usage involving all with the fraction one half (or the half ) is prevalent elsewhere in the text, including a nearby example in Helaman 4:10:
In four of these cases, the all is followed by a reduced relative clause (where the direct object relative pronoun that is lacking), and as a result the all cannot be deleted in those four examples. But in the two other examples, the all is followed by a noun phrase (as here in Helaman 4:16). In those cases, the all is not required. For instance, for the second case in Mosiah 19:15, we can see the choice in the use of all; the all appears before their precious things, but not before their gold or their silver:
Since in Helaman 4:16 the text will work either with or without the all, there would have been no motivation for Oliver Cowdery to edit the text here. Undoubtedly, 𝓞 had the all.
Summary: Accept Oliver Cowdery’s virtually immediate insertion of all in 𝓟 for Helaman 4:16 (“and the one half of all their lands”); 𝓞 undoubtedly had the all.