Isaiah 6:2 (King James Bible) above it stood the seraphims
The original Book of Mormon text uses the double plural seraphims, just like the King James Bible. In Hebrew, the -im ending makes seraph plural. But for English speakers, seraphim doesn’t seem plural, thus the tendency to add the -s ending to the Hebrew plural seraphim to form a double plural, seraphims. This same double plural is found later in this chapter:
The 1920 LDS edition replaced the King James seraphims with the correct Hebrew plural form, seraphim. The critical text will, of course, restore the original Book of Mormon reading in both cases (in agreement with the King James reading).
Summary: Maintain the double plural seraphims in 2 Nephi 16:2, 6.