The expression “this is ” is grammatically nonstandard; thus it is not surprising that Joseph Smith, in his editing for the 1837 edition, changed the expression to the plural “these are ” (although he only marked the change from is to are in 𝓟). The nonstandard expression is fairly rare in the earliest text. In fact, there is only one other example of its kind in the text; and again, as we might suspect, Joseph eliminated it in his editing for the 1837 edition (and once more he marked only the change from is to are in 𝓟):
The critical text will restore the original nonstandard usage in both of these cases since it appears to have been intended.
Summary: Restore the two examples of the nonstandard “this is ” in Mormon 7:1 and Ether 2:15, the original reading in both cases.