In the original manuscript, the preposition of is found at the head of the relative clause, thus “of whom he had labored with so much diligence to preserve”. The of was copied into the printer’s manuscript and set by the 1830 compositor. Elsewhere in the original text, we have no cases of the verb preserve taking a prepositional phrase rather than a direct object noun phrase as its complement (see, in particular, the discussion regarding the phrase “preserved unto himself ” in Jacob 5:74). It is possible that this extra of in Alma 51:14 is a scribal error, especially given the immediately preceding occurrences of of (“because of the stubbornness of those people”). Joseph Smith removed this preposition in his editing for the 1837 edition.
Nonetheless, as discussed under 2 Nephi 3:14, there are similar instances in the earliest text of an unexpected of at the head of a relative clause. Despite their difficulty, these examples do not seem especially objectionable. The critical text will therefore restore in Alma 51:14 the original instance of of in the relative clause “of whom he had labored with so much diligence to preserve”; nonetheless, the possibility remains that this of is an error resulting from the preceding occurrences of of.
Summary: Restore in Alma 51:14 the original of in “of whom he had labored with so much diligence to preserve”; although unusual, this of may have been in the original text.