The printer’s manuscript has the singular affliction, which the 1830 typesetter changed to the plural afflictions. Since Oliver Cowdery frequently added or dropped plural s ’s, the singular reading in the printer’s manuscript could be a copy error on Oliver’s part. Or the 1830 typesetter may have incorrectly added the s. Interestingly, the 1905 LDS missionary edition restored the singular affliction, but probably unintentionally since 𝓟 was not available for that edition.
There are four other cases of “the affliction(s) of X” in the text, and in each case the earliest text has the plural afflictions:
These four examples suggest that the 1830 emendation in Alma 17:30 was correct.
But more generally, there is considerable evidence in the text that the noun affliction can occur in either the singular or the plural, although the plural is preferred overall. The critical text will therefore determine the number for each case of affliction(s) on the basis of the earliest textual evidence. The discussion under Mosiah 9:3 provides considerable evidence for this decision; also see under 1 Nephi 16:35 for additional examples of the variation regarding affliction(s). Thus the critical text will accept the singular affliction in “the affliction of those which he termed to be his brethren”, the reading of the printer’s manuscript for Alma 17:30. Even though the singular usage here is unique for this particular expression, this does not mean that the singular is an error. There is nothing inherently wrong with the singular affliction in “when he saw the affliction of those which he termed to be his brethren”.
Summary: Restore in Alma 17:30 the singular affliction in “the affliction of those which he termed to be his brethren”; although this instance of the singular is unique for the expression “the affliction(s) of X”, general usage in the text permits both singular affliction and plural afflictions.