The of here is supralinearly inserted with heavier ink flow in the printer’s manuscript. The original manuscript is not extant here, so one cannot be sure whether the insertion is correcting to 𝓞 or represents editing on the part of the corrector. In fact, it is difficult to tell who the corrector is. The of and its accompanying insert mark do not quite look like Oliver Cowdery’s, so perhaps the 1830 typesetter or someone else corrected the text here. In that case, the correction would seem to be an editorial one.
The first question we ask here in 1 Nephi 1:16 is whether this relative clause needs a preposition (presumably of ). This issue comes up fairly frequently in the text, and virtually every example has its own peculiarities. In analyzing this example, we first consider the general phrase “to make an account” and note that when it has a complement, it is always a prepositional phrase headed by of. In all there are eight other cases, of which three are nearby:
Thus we expect the preposition of in the relative clause in this same passage.
The second question we ask is whether the preposition of should come at the beginning or at the end of the relative clause. With other verbs, either position is possible, yet in nearly all cases the clear majority are found at the head of the relative clause. (For a complete discussion regarding prepositional position, see relative clauses in volume 3.) This strong tendency suggests that the corrected reading in 𝓟 is probably the original reading, even if that correction is due to editing and the of happened to be missing from 𝓞.
Summary: Retain the corrected text “of which I shall not make a full account” since the preposition of is expected in the expression “to make an account” and its placement at the head of the relative clause is the more probable.