Oliver Cowdery initially wrote would here in 𝓟; then virtually immediately he crossed out the would and wrote could supralinearly (there is no difference in the level of ink flow for the correction). Elsewhere in the text, there are four instances of “would know” and one of “could know” (marked below with an asterisk):
So either reading is theoretically possible here in Helaman 9:41; the critical text will follow the corrected reading in 𝓟, “he could not know of all things”.
Summary: Accept in Helaman 9:41 Oliver Cowdery’s correction in 𝓟 of “would not know” to “could not know”; this correction undoubtedly represents the reading in 𝓞, no longer extant here.