Oliver Cowdery initially wrote simply “in body and in mind”; then he supralinearly inserted both before this conjunctive expression. The level of ink flow is the same, so the change appears to be a correction to the reading of the original manuscript (no longer extant here). There is only one other occurrence of body conjoined with mind, in Mosiah 2:11, and this reads without both (as well as without the repeated in): “subject to all manner of infirmities in body and mind”. The infrequency of the combination of body and mind argues that in Alma 17:5 there would have been very little, if any, motivation for Oliver to have consciously emended “in body and in mind” to “both in body and in mind”.
There are 13 occurrences in the text of “both in X and in Y”, of which 12 form a contrastive pair; we have five basic types:
(1) both in word and in deed 2 Nephi 3:24
(2) both in the east and in the west 2 Nephi 29:11
both in the land south and in the land north Helaman 6:9
both in the north and in the south Helaman 6:12 (twice)
both in the land north and in the land south 3 Nephi 1:17
(3) both in heaven and in earth Jacob 7:14, Mosiah 4:9 (twice),
Alma 22:10
(4) both in body and in mind Alma 17:5
(5) both in their front and in their rear 3 Nephi 4:25
There is a sixth type of “both in X and (in) Y” but without the repeated preposition:
(6) both in towns and villages Mormon 4:22
There is only one example of “both in X and in Y” that does not form a contrastive pair: “both in buildings and in gold and in silver and in raising grain and in flocks and herds and such things which had been restored unto them” (Ether 10:12). In any event, the use of both in Alma 17:5 is consistent with usage elsewhere in the text.
Summary: Accept Oliver Cowdery’s virtually immediate insertion of both in Alma 17:5 since this predeterminer very probably represents the reading of the original manuscript.