The original manuscript is not extant for the phrase “in his wisdom”, but there is definitely space for the determiner his between surviving portions of the original manuscript. In the printer’s manuscript, Oliver Cowdery initially wrote the text without the his, but later he inserted the his supralinearly. The level of ink flow for the correction is somewhat heavier, which suggests that here Oliver redipped his quill before correcting 𝓟 or that the correction was done later when proofing 𝓟 against 𝓞. Despite Oliver’s correction in 𝓟, the 1830 compositor set simply “in wisdom”, without the his. The 1908 RLDS edition, based on the correction in the printer’s manuscript, restored the his; but in the subsequent 1953 RLDS edition, the his once more dropped out. This difficulty in keeping the his is undoubtedly due to the phrase “in wisdom” that occurs in the preceding sentence (“yea in wisdom all that he seeth fit that they should have”).
Elsewhere in the text there are two occurrences of the verb counsel with wisdom, and they both take the form “in wisdom”—that is, without a determiner:
Nonetheless, there are a number of cases involving other verbs where the text has “in wisdom” and the reference is to the Lord’s wisdom:
So the determiner his is definitely possible in Alma 29:8, and the earliest textual sources (the corrected reading in 𝓟 and the spacing between fragments of 𝓞) support the his. In addition, there would have been no motivation for adding the his except in an attempt to get 𝓟 to agree with 𝓞. The critical text will therefore restore the his here in Alma 29:8.
Summary: In accord with the earliest textual source (the printer’s manuscript), restore the phrase “in his wisdom” in Alma 29:8 (“the Lord doth counsel in his wisdom”).