The original manuscript here reads “such wickedness and infidelity”. When copying from 𝓞 into 𝓟, Oliver Cowdery misread infidelity as iniquity, undoubtedly because the text otherwise has only instances of iniquity and its plural iniquities (for a total of 226 instances). There are no other instances in the Book of Mormon text of the word infidelity (nor in the King James Bible, for that matter). The 1981 LDS edition, by reference to 𝓞, restored the original infidelity here.
Summary: Maintain in Alma 31:29 the original but unique occurrence in the text of the noun infidelity.