Here the text starts out with two cases of “thou shalt”, followed by a conjoined predicate that in the earliest text begins with shall rather than the grammatically expected shalt: “thou shalt serve me … and [thou] shall gather together my sheep”. It is possible that the shall is an error, perhaps influenced by two occurrences of shall in the subsequent text: “he … shall be my sheep and him shall ye receive”. It is also possible that the preceding base form go (“and go forth in my name”) may have led to the use of the base form of the modal verb shall in the following conjoined predicate (“and shall gather together my sheep”). The 1879 LDS edition made the change to the expected shalt, which has been retained in the LDS text. The RLDS text has maintained the earlier shall.
As discussed under Mosiah 12:11, there are other cases in the earliest text where thou takes the base form of the verb rather than the expected archaic ending -est or, in the case of modals like shall and will, the ending -t (that is, shalt and wilt). Since the use of the base form may be intended, the critical text will follow the earliest reading here in Mosiah 26:20. Also see the discussion regarding the use of may rather than mayest nearby in Mosiah 26:11.
Summary: Restore in Mosiah 26:20 the reading in 𝓟, the earliest extant reading, where the subject thou takes the base form of the modal verb shall in a conjoined predicate.