In these two verses, the original text has Abinadi addressing the priests of Noah with the singular pronoun thou four times and the possessive pronoun thine once, not the expected plural pronoun ye or you or the possessive pronoun your. There is a similar instance of this usage in verse 27 (“what teachest thou this people”), as well as elsewhere in the text (see the discussion under 1 Nephi 3:29). In his editing for the 1837 edition, Joseph Smith substituted the corresponding plural forms for each of these five cases here in Mosiah 12:30–31.
This particular passage is especially interesting in that the five instances of the second person singular (thou and thine) are intermixed in the original text with one example of you and three of ye. Clearly, variation is possible, even within the same sentence (“ye shall be smitten for thine iniquities”). We can also tell that the use of thou is intended since in each case of thou the corresponding verb takes the -est ending: “knowest thou … thou knowest ... knowest thou ... sayest thou”. For a complete listing of all examples where the second person singular pronoun refers to more than one person, see under thou in volume 3. The critical text will restore all instances of this nonstandard usage whenever it is supported by the earliest textual sources.
Summary: Restore the five uses of the second person singular pronoun in Mosiah 12:30–31 (four cases of thou and one of thine); correspondingly, the -est inflectional ending will also be restored to the verbs that take thou as subject.