Here in Mosiah 18:10 and 18:11, the 1837 edition changed the plural desires to the singular desire, undoubtedly because of the preceding singular subject this (in both cases) and the singular verb form is (in the second case). The first change was also marked by Joseph Smith in the printer’s manuscript. This same grammatical change has been made in the 1830 and 1837 editions for a number of similar occurrences of the plural desires:
In all of these cases, the text uses a singular pronoun (such as it or this) to directly refer to the plural desires. Thus the occurrences in Mosiah 18:10–11 of “if this be the desires of your hearts” and “this is the desires of our hearts”, despite the nonstandard grammar, are quite consistent with usage found elsewhere in the original text. The critical text will restore the plural desires in all these cases since such usage is clearly intended. For further discussion, see under subjectverb agreement in volume 3.
It should also be pointed out that elsewhere the text refers to “the desires of the heart”, not “the desire of the heart”:
Note that it makes no difference whether the text is referring to one person or many: we consistently get the plural desires with heart(s). Thus the original use of “the desires of your hearts” and “the desires of our hearts” in Mosiah 18:10–11 is consistently supported by usage elsewhere.
Summary: Restore the original instances of the plural desires in Mosiah 18:10–11 (“if this be the desires of your hearts” and “this is the desires of our hearts”); the plural desires is supported by usage elsewhere in the text.