The very first occurrence of the name Morianton in the original manuscript is actually spelled Morionton. In fact, every extant occurrence in 𝓞 of this name is spelled with three o ’s. In the book of Alma (from Alma 50:25 through Alma 59:5), this name occurs 20 times and refers either to the land or city of Morionton or to the Nephite leader Morionton (after whom the land and city were presumably named). Of these 20 occurrences, 13 are sufficiently extant in 𝓞 to determine that the name was spelled there as Morionton, including the first occurrence.
In the printer’s manuscript, Oliver Cowdery changed the original spelling to Morianton—that is, the second o was changed to an a. Of the 20 occurrences in Alma, Oliver wrote Morianton in 𝓟 the first 18 times; for the last two (the second occurrence in Alma 55:33 and the very last occurrence, in Alma 59:5), he wrote the original Morionton. These last two occurrences of Morionton thus indicate that Oliver probably wrote Morionton in 𝓞 all the way through the book of Alma. Based on this evidence, the Nephite leader and the land and city should be spelled as Morionton.
Difficulties arise when we compare the name Morionton with the name Morianton as used in the book of Ether and with two place-names that are similar to Morianton:
For these three other names, nothing is extant from 𝓞. As for the seven occurrences of these three names, 𝓟 consistently has the a spelling, not the o. The similarity between Moriancumer and Coriantumr suggests that the a vowel is correct in Moriancumer.
Hugh Nibley has argued that the original people of Zarahemla, the Mulochites (the Mulekites of the standard text), adopted some Jaredite names. For this proposal, see his discussion under “Nephites with Jaredite Names” on pages 243–248 of Lehi in the Desert and the World of the Jaredites (Salt Lake City, Utah: Bookcraft, 1952). Following Nibley’s argument, if the Nephite leader’s name Morionton is the same as the Jaredite king’s name, then (assuming there has been no phonological change) one could claim that the five occurrences of Morianton in the book of Ether (but found only in 𝓟) should be emended to Morionton (which is consistently supported by 𝓞 in the book of Alma). Ultimately, the critical text must rely on the earliest textual sources for determining the spelling of names. And the evidence from the manuscripts supports the spelling Morianton for the name of the Jaredite king, as well as Moriancumer and Moriantum (at least with respect to the a vowel). Nor must we assume that similar names should be reduced to identical names. There are a number of names in the Book of Mormon text that vary minimally from one another in spelling. See, for instance, the discussion under Alma 47:5 regarding the separate names Oneidah, Onidah, and Onihah; also see the discussion under 4 Nephi 1:47 regarding the four names Ammaron, Ammoron, Amaron, and Amoron. Apparently, we have two distinct names, Morionton (in Alma) and Morianton (in Ether), based on the earliest extant spellings (in 𝓞 for Morionton and in 𝓟 for Morianton).
Summary: Change the spelling of the Nephite leader (and the city and land presumably named after him) to Morionton; there are 13 sufficiently extant occurrences of this name in the original manuscript for Alma 50–59, and all are spelled with an o in the middle of the name; on the other hand, the a vowel in the middle of names like Moriancumer, Moriantum, and the Jaredite king Morianton will be retained (in accord with their spellings in the earliest extant source, namely 𝓟).