Except for one possible exception, the Book of Mormon text systematically distinguishes between doeth and doth. The first, doeth, is used for the main verb do, and the second, doth, for the helping verb do, as in “the man that doeth this” (Mosiah 2:37) versus “as a child doth submit to his father” (Mosiah 3:19). The one passage where there is some question as to whether the verb form should be doeth or doth is found in Alma 3:19 (see the discussion there).
Here in Mosiah 4:18, Oliver Cowdery initially wrote doth rather than the correct doeth. Almost immediately he corrected doth to doeth by supralinearly inserting the e (there is no change in the level of ink flow). Later on in the book of Alma, the same initial error was made in 𝓟, although by a different scribe (in this case, the unknown scribe 2 of 𝓟):
Once more, the correction is virtually immediate.
In general, the critical text will follow the earliest textual reading in determining whether the third person singular present tense form for the verb do should be doth or doeth. Except for the one possible case of Alma 3:19, such a procedure makes the text fully regular in distinguishing between doeth and doth. Here in Mosiah 4:18, it appears that the corrected reading in 𝓟 represents the original reading. For a complete discussion, see under infl al endings in volume 3.
Summary: Maintain in Mosiah 4:18 the corrected doeth rather than the initially written doth; the correction is virtually immediate and appears to be the reading of the original manuscript.