Here we have another case of subject-verb disagreement in the original text (“all things ... which is expedient”). For the 1837 edition, Joseph Smith edited the singular is to the plural are. The critical text will follow the earliest reading, even if it is ungrammatical for standard English. See the discussion under 1 Nephi 17:30 as well as the more general discussion under subject-verb agreement in volume 3.
Summary: In accord with the reading of the earliest text, restore in 2 Nephi 2:27 the singular is in the relative clause “which is expedient”, even if its antecedent (“all things”) is in the plural.