The name Melchizedek was originally spelled in the Book of Mormon manuscripts in accord with its common pronunciation, the nonstandard /melke zßdIk/ rather than the standard /melkI zßdek/. The 1830 typesetter provided the Old Testament (King James) spelling Melchizedek for the name; for the 1908 RLDS edition, the New Testament (King James) spelling Melchisedec was substituted into the RLDS text.
Overall, the name Melchizedek is found five times in the Book of Mormon text (all in Alma 13:14–18); and in each case the textual history is identical except in 𝓟, where the name is spelled Melchesidek for the three last instances:
Melchezidek in 𝓟:
Melchesidek in 𝓟:
It was perhaps this latter spelling in 𝓟 that led the editors for the 1908 RLDS edition to adopt the New Testament spelling Melchisedec, especially since 𝓟 was extensively used in producing that edition.
As noted in the discussion under 1 Nephi 11:13, Joseph Smith did not spell out known biblical names as he dictated the text to his scribe. Even if the scribe didn’t know how to spell a given biblical name, it was presumed that the typesetter would correct the spelling for biblical names (which is what happened for the name Melchizedek here in Alma 13). The critical text will retain the Old Testament spelling of the name, although Joseph Smith and his scribes probably pronounced this name differently than the standard spelling would indicate.
Summary: Maintain the standard spelling Melchizedek in Alma 13:14–18.