Here we have two occurrences of “office of X”. Since the use of being is not necessary, there seems to be little editorial motivation for Oliver Cowdery to have added this word to the text for one of these cases. The insertion of being is probably the result of Oliver’s proofing of the printer’s manuscript against the original manuscript.
The use of the verb being is acceptable since “the office” is followed by the prepositional phrase “over the church”, which implies some kind of predication (as if the text could have read “the office of high priest presiding over the church”). The second occurrence of “office of X” does not involve such a predicative context, and thus no verbal element is found there.
Summary: Accept in Alma 4:18 the inserted being, Oliver Cowdery’s correction in 𝓟 that was undoubtedly based on 𝓞.