There are many cases in the original text where agreement between subject and verb is nonstandard, especially when the verb is be. Here we have an example where a conjunct of three attributes of God (his power, goodness, and mercy) is assigned the singular is. In his editing for the 1837 edition, Joseph Smith changed the verb to the plural are. In general, cases involving subject-verb agreement are discussed as a group in volume 3 under subject-verb agreement.
Summary: The critical text will maintain the original examples of nonstandard subject-verb agreement.