The printer’s manuscript shows some variation here. Oliver Cowdery wrote cumoms for the first instance of the word, but later in the verse he wrote comoms for the second instance. Based on the assumption that the scribe took care to correctly spell the first occurrence of a Book of Mormon name or word, we can assume that cumoms is the correct reading here in Ether 9:19. The 1830 typesetter assumed as much since he set both instances as cumoms.
Of course, one could imagine that the cu spelling for cumoms was an error based on the cu in the immediately preceding cureloms. But then one could also argue that the om in the first syllable of comoms was caused by the combined influence of the final om in cureloms and the immediately following om in the second syllable of cumoms.
Elsewhere in the text, we get Book of Mormon names beginning with either cum or com:
In fact, there are two cases where scribe 2 of 𝓟 wrote Cumorah as Comorah (in Mormon 6:5, 11), which provides support here in Ether 9:19 for the change from cu to co in the second instance of cumoms (although here in the book of Ether the scribe in 𝓟 was Oliver Cowdery, not scribe 2).
Either cumoms or comoms is theoretically possible in Ether 9:19. The critical text will follow the spelling of the first occurrence in 𝓟, cumoms.
Summary: Maintain cumoms as the correct spelling in Ether 9:19 since it is the spelling of the first instance of the word.