Here in the printer’s manuscript, Oliver Cowdery initially wrote “hath sent me”, which is the language he expected. Somewhat later, with heavier ink flow, Oliver corrected this perfectly acceptable reading to the rather unusual “hath cause to send me”, although it should be noted that he neglected to insert the infinitival to (he actually wrote “hath cause send me”). The 1830 typesetter added the necessary to in the 1830 text, and Joseph Smith himself later inserted the to in the printer’s manuscript when he edited the text for the 1837 edition.
Elsewhere there are 49 examples where the text refers to the Lord sending someone, but none of the Lord “having cause” to send someone. Thus there would have been no motivation here in Mosiah 12:29 for Oliver Cowdery to have edited the text from “hath sent” to “hath cause to send”. It is true that in the preceding clause Abinadi used the verb cause when he asked why the priests “cause this people to commit sin”. Of course, the noun cause in “the Lord hath cause to send me” is used quite differently. Ultimately, there was no need to edit the text here in Mosiah 12:29 except that the original manuscript must have read “hath cause to send”.
The use of the phrase “to have cause to do something” is fairly frequent elsewhere in the Book of Mormon text:
All the evidence argues that Oliver Cowdery’s later correction in Mosiah 12:29 is the reading of the original text.
Summary: Retain in Mosiah 12:29 Oliver Cowdery’s corrected reading in 𝓟, “the Lord hath cause to send me”; this correction undoubtedly follows the reading of the original manuscript, no longer extant for any of the book of Mosiah.