For both occurrences of this name here in Alma 47:5, 𝓞 and 𝓟 have the spelling Oneidah, not Onidah. The 1830 compositor set this name as Onidah, which is the spelling used earlier in the text for the hill Onidah:
𝓞 is not extant in Alma 32:4 for Onidah, but 𝓟 and all the printed editions have that spelling for the name of the hill that Alma preached on. This hill was in the land of Antionum (Alma 31:3), the place where the Zoramites lived. The name Oneidah in Alma 47:5 does not refer to the name of the mount that Lehonti and his army fled to (instead, the name of that mount is Antipas, mentioned later in verses 7, 9–10). Mount Antipas is in the land of Nephi and is not the same as the hill Onidah in the land of Antionum. In fact, Oneidah does not directly refer to a hill or mount. Thus there is no reason to suppose that Oneidah (which was where “the place of arms” was located) should necessarily be spelled the same as Onidah (“the hill Onidah”). It is also worth noting that there is a third name in the text that is similar to Onidah, namely Onihah (a city mentioned in 3 Nephi 9:7). Frequently, Book of Mormon names can be very similar, differing by only a single letter. In each case, the critical text will rely on the earliest textual sources for determining the spelling of names, thus Oneidah here in Alma 47:5, Onidah in Alma 32:4, and Onihah in 3 Nephi 9:7).
It should also be noted that the 1830 compositor’s replacement of Oneidah with Onidah here in Alma 47:5 could have been influenced by the name of the Iroquois tribe Oneida, but only if the compositor thought the name of the tribe was spelled Onida (or perhaps Onidah, with a word-final silent h). The change from Oneidah to Onidah moves the spelling away from Oneida, the standard spelling of the Iroquois tribe. But since Oneida is pronounced /ounaidß/, Onida (or Onidah) is an alternative spelling. There is, for instance, the name of a town in South Dakota, Onida, which is “a misspelling of Oneida, the New York hometown of the first settlers who arrived here in 1883”; see page 152 of Linda Hasselstrom, Roadside History of South Dakota (Missoula, Montana: Mountain Press Publishing Company, 1994). And Oneida, the company that manufactures flatware, can be found on internet searches even if its name is misspelled as Onida.
Summary: Based on the reading in both manuscripts, the correct name for the location of the place of arms in Alma 47:5 is Oneidah, not Onidah.