In this conjoining of predicates, Oliver Cowdery initially omitted the hath before the second predicate (thus initially writing “and caused that your hearts should be filled with joy”). Almost immediately Oliver inserted the hath supralinearly, although he ran out of ink while doing so, and so he redipped his quill and overwrote the weakly written ath of the inserted hath with heavier ink flow. When Oliver came to the third predicate, he included the hath (thus “and hath caused that your mouths should be stopped”). The occurrence of the hath in the third predicate supports the occurrence of a parallel hath in the preceding predicate.
Other examples in king Benjamin’s discourse show that there is a preference for the text to repeat the perfect auxiliary have in conjoined predicates:
Thus there is considerable support for multiple repetition of the hath in Mosiah 4:20.
Here in Mosiah 4:20, the 1837 edition replaced all three instances of hath with has, the verb form that we expect in modern English. (Only the third instance of the change was actually marked by Joseph Smith in his editing of 𝓟 for that edition.) Such editing, of course, is irrelevant to the issue of whether the perfect auxiliary verb should be repeated in this conjunctive structure.
Summary: Maintain in Mosiah 4:20 Oliver Cowdery’s supralinearly inserted hath in the conjoined predicate “and hath caused that your hearts should be filled with joy”; such repetition is characteristic of the language in king Benjamin’s discourse.