Here is another case in the original text where agreement between subject and verb is nonstandard. (See 1 Nephi 1:14 for an earlier example.) In this instance, the subject is clearly plural (“the tender mercies”), but the intervening prepositional phrase (“of the Lord”) favors the singular is (as if the text simply read “the Lord is”). In his editing for the 1837 edition, Joseph Smith marked in 𝓟 the editorial change to the plural are. The critical text will, of course, restore the original subject-verb disagreement. In general, cases of subject-verb agreement will be discussed as a group in volume 3 under subject-verb agreement.
Summary: Retain in 1 Nephi 1:20 the original singular verb form is, even though its subject noun phrase has the plural mercies as its head.