In this passage, we have two instances of “X’s whole soul(s)”, with both occurring in a plural context. In the first instance, the earliest reading is “your whole souls hath power”. In his editing for the 1837 edition, Joseph Smith edited the third person singular hath to the plural have (thus giving the standard “your whole souls have power”). But in the actual 1837 edition, this phrase was set as “your whole souls has power”, a clearly unacceptable reading. The 1840 edition finally implemented the standard “your whole souls have power”, which has been retained in the RLDS textual tradition. On the other hand, the incorrect “your whole souls has power” was retained in the LDS text until the 1906 edition. There the text was corrected to the singular “your whole soul has power”. In other words, the singular has was kept and the subject noun souls was edited to the singular.
In the second instance of “X’s whole soul(s)”, the earliest text read in the singular: “with all your whole soul”. This reading was maintained in the early editions and has continued throughout the RLDS textual tradition. But in the 1852 LDS edition, the singular was changed to the plural (“with all your whole souls”), perhaps because of the plural context and because at that time in the LDS text the instance of “X’s whole soul(s)” in the previous verse still read in the plural (“your whole souls has power”). Here in verse 21, the plural reading has continued in the LDS text. Ultimately, the LDS text ended up reversing the number for both occurrences of “X’s whole soul(s)” in Mosiah 2:20–21.
The original use of the plural souls in verse 20 (the first instance) is perfectly acceptable. Elsewhere in the text, there are two occurrences in a plural context of “X’s whole soul(s)”; one has the singular soul and the other the plural souls:
The second of these (Omni 1:26) thus directly supports the original plural reading in Mosiah 2:20. And actually the first one (2 Nephi 25:29), with its singular reading, supports the original singular reading in Mosiah 2:21. Note that in the larger conjunctive structure for 2 Nephi 25:29, we have “with all your might mind and strength and your whole soul”, with its use of all at the beginning of the first nominal conjunct. Admittedly, the use of both all and whole in “with all your whole soul” in Mosiah 2:21 is somewhat redundant, but that redundancy is not alleviated by the use of the plural form “with all your whole souls”.
The critical text will therefore retain the earliest readings for “X’s whole soul(s)” in Mosiah 2:20–21: namely, plural souls in verse 20 and singular soul in verse 21. In addition, the first instance will retain the use of the biblical hath, which frequently occurred with third person plural subjects in the original text of the Book of Mormon.
Summary: Restore “your whole souls hath power” in Mosiah 2:20 and “with all your whole soul” in Mosiah 2:21; these original readings can, in each case, be supported by usage elsewhere in the text.