Here the 1841 British edition accidentally spelled Shemlon as Shemlom. This error was not continued in the subsequent 1849 LDS edition. This is the first occurrence of the place-name Shemlon in the text. All other occurrences of Shemlon (five of them) are invariant throughout the textual history.
The error Shemlom was probably influenced by preceding occurrences of the place-name Shilom (there are seven preceding occurrences of Shilom, from Mosiah 7:5 through Mosiah 9:14). David Calabro also points out (personal communication) that many other Book of Mormon names end in -om, including three more place-names: Nahom (1 Nephi 16:34), Shimnilom (Alma 23:12), and Ablom (Ether 9:3).
Another possible explanation is that this change of a final n to an m was due to the influence of the preceding labial m within the name Shemlon itself. Another name that may have been influenced by such an assimilatory tendency is the name Shiblon, which has sometimes been replaced by Shiblom in the text. The preceding labial in this case is the stop b. We have two explicit instances of this error in the manuscripts, both initially made by Oliver Cowdery and immediately corrected by him (by means of erasure):
These initial errors in the manuscripts have some bearing on the question of whether this name should read Shiblon or Shiblom in the book of Ether. For discussion of that issue, see under Ether 1:11–12.
Summary: Maintain Shemlon, the consistent manuscript spelling for this place-name.