Here in the printer’s manuscript, Oliver Cowdery initially wrote what he expected: “the Lord hath sent him”. Oliver crossed out sent him and supralinearly inserted the correct verb form, spoken (the level of ink flow is unchanged), and then he wrote the it inline after the crossed-out sent him. The correction was therefore immediate. Since either reading (“sent him” or “spoken it”) is possible, we accept the corrected reading in 𝓟 as the reading of the original manuscript.
Summary: Accept in Mosiah 12:12 Oliver Cowdery’s corrected reading in 𝓟: “and he pretendeth the Lord hath spoken it”.