Twice here in the printer’s manuscript, Oliver Cowdery initially wrote whosoever (listed above under 1 and 2), but later, probably when he proofed 𝓟 against 𝓞, he crossed out both instances of the extra ever (the level of ink flow for the crossouts is somewhat heavier). 𝓞 is sufficiently extant in the first case that we can determine that the word is the shorter whoso. For the third case of whoso in this verse, 𝓞 is fully extant and reads whoso, not whosoever. In that third case, Oliver wrote whoso without error in 𝓟.
We also note here that the second instance of whoso originally took the plural verb form were. The 1837 edition changed this to the singular was, probably as a result of Joseph Smith’s editing for that edition (although the change was not marked in 𝓟). As discussed under 1 Nephi 17:48, the original text allows for either a singular or plural interpretation for the generic pronoun whoso(ever). Here in Ether 10:6, the critical text will restore the original plural were.
As explained under Ether 9:33, the Book of Mormon text prefers whoso in the book of Ether. Starting near the end of Helaman and going up to the end of Mormon, there is considerable variation between whosoever and whoso. In that section of the text, there is one more instance in 𝓟 where Oliver Cowdery initially wrote whosoever instead of the correct whoso:
Summary: Maintain in Ether 10:6 the three instances of whoso, two of which are extant in 𝓞; for two of the three cases, Oliver Cowdery initially wrote whosoever in 𝓟 but later corrected these to whoso when he proofed 𝓟 against 𝓞; the plural were for the second instance of whoso will also be restored.