Here in 𝓟, Oliver Cowdery initially wrote “he could not know all things”. Somewhat later, with weaker ink flow, he supralinearly inserted of after know. Oliver’s correction may have occurred when he proofed 𝓟 against 𝓞. As explained under Alma 17:16, there has been a tendency in the history of the text to omit the of from instances of “to know of X” (but not to insert the of in instances of “to know X”). In either case, we follow the earliest textual sources in determining whether the original text read “to know X” or “to know of X”.
Elsewhere in the text, there are ten instances of “to know all things”, but no others of “to know of all things”. The predominance of the form without the of may have led Oliver Cowdery to initially omit the of here in Helaman 9:41.
Summary: Maintain in Helaman 9:41 the use of of in “he could not know of all things”, the corrected reading in 𝓟 (the earliest extant reading).