Here the original manuscript incorrectly spells Ammon as Ammen. The name Ammon occurs 53 times in the extant text of 𝓞, at least for the last vowel of the name. (Included in this number are four extant occurrences of the name in the headings that the scribe wrote at the top of each page of 𝓞.) In 51 of these cases, from Alma 19:11 through Alma 58:39, 𝓞 reads Ammon without variation for the o vowel. Thus the spelling Ammen here in Alma 24:5 is undoubtedly an error. The same error in 𝓞 occurred later in the book of Alma, but in this instance, Oliver caught his error and corrected it virtually immediately, overwriting the e with the correct o (there is no difference in the level of ink flow):
On the other hand, Ammon is never misspelled in 𝓟. The critical text will maintain the spelling Ammon.
Summary: The name Ammon was misspelled as Ammen two times in 𝓞 (and one of these was a momentary error); the earliest extant spelling of the name is Ammon.