We do not have the original manuscript here, but it appears that Oliver Cowdery wrote (at least initially) “in the which” in 𝓞 since spacing considerations between extant fragments give ample room for the article the. However, it is quite possible that in the original manuscript Oliver corrected this example of “in the which” to “in which” by crossing out the the. While copying this passage from 𝓞 into 𝓟, Oliver had completed only the beginning strokes for the the when he caught himself and turned the uncrossed t and the beginning stroke for the h into a capital W. He then continued writing the rest of the word which inline. The immediacy of this correction suggests that Oliver himself tended to write “in the which”, even when “in which” was the correct reading.
Elsewhere in the text, there were originally 56 occurrences of “in the which”, although exactly half of these were edited out by Joseph Smith for the 1837 edition. On the other hand, the original text (judging from the earliest textual sources) had at least four other occurrences of “in which”. See the discussion regarding “in (the) which” under 1 Nephi 3:2 as well as more generally under in the which in volume 3. The important point here is that Oliver Cowdery himself did not show any inclination to remove the definite article the in this phrase in any of the 56 original occurrences of “in the which” in the text; in fact, the printer’s manuscript here in Alma 23:15 suggests that Oliver preferred or was quite used to “in the which”—and that he quite probably wrote “in the which” initially in the original manuscript for this passage as well but then corrected it to “in which”. The shorter form “in which” is apparently correct here in Alma 23:15, although elsewhere in the original Book of Mormon text the clear majority of cases have the the. The critical text, for each case of “in (the) which”, will follow the earliest textual evidence in determining whether the the was there or not.
Summary: Maintain Oliver Cowdery’s corrected “in which” in Alma 23:15; although the text usually reads “in the which”, there is evidence in a handful of cases that the original text read “in which”.