Isaiah 2:2 (King James Bible) and it shall come to pass in the last days that the mountain of the LORD’s house shall be established in the top of the mountains and shall be exalted above the hills and all nations shall flow unto it
The 1953 RLDS edition accidentally, it would seem, replaced the plural hills with the singular hill, perhaps under the influence of the it at the end of the following clause (“and all nations shall flow unto it”), although the it, of course, refers to the mountain of the Lord’s house. The King James Bible (as well as the original Hebrew) has the plural hills.
Summary: Maintain the plural hills in 2 Nephi 12:2, the reading of the earliest textual sources and the King James Bible.