There are two minor changes here. First, in the 1858 Wright edition, the original and at the beginning of the first clause was accidentally omitted. Second, in the 1907 LDS edition as well as in the 1858 Wright edition, the definite article the was inserted before mountains (but not before whirlwinds or buildings). Neither of these two changes persisted in the text. One possible motivation for adding the the is that earlier in the Isaiah quotation in 2 Nephi 12–24, the text refers to “the mountains” twice:
However, references to destruction in the Book of Mormon typically refer to mountains without using the definite article the:
Thus the original usage in 2 Nephi 26:5 is wholly appropriate.
Summary: Maintain the earliest reading in 2 Nephi 26:5: “and mountains shall cover them”—that is, with an and at the beginning and without a the before mountains.