The use of the preposition upon in the phrase “to the borders upon the seashore” seems odd. Instead, we expect either the preposition by or of. Elsewhere the text has only these two prepositions in phrases like this one; the preposition is normally by (12 out of 14 cases), but there are two instances with of (each marked below with an asterisk):
In addition to the case here in Ether 14:12, two of the above cases involve motion “to the borders” (Alma 50:9 and Ether 14:26). In the earliest text, both of these read “to the borders by the seashore”, which argues that Ether 14:12 may have also originally read that way instead of “to the borders upon the seashore”.
The original manuscript is not extant for Ether 14:12; the printer’s manuscript and the printed editions read “to the borders upon the seashore”. Yet the preposition upon could well be an error from the following verse, where the phrase “upon the seashore” occurs. In fact, this phrase is found immediately underneath on the following line in 𝓟 (and would have been immediately underneath on the following line in 𝓞). In fact, the last part of both lines in 𝓟 are virtually identical, and Oliver Cowdery could have easily let his eye stray down a line when he copied the text from 𝓞 into 𝓟. The first two lines on page 451 of 𝓟 originally read as follows (prior to the 1830 typesetter’s adding punctuation in pencil to this page of the manuscript):
In other words, here in Ether 14:12 the original manuscript could have read “to the borders by the sea shore”, just as it did later in verse 26 of this chapter in the earliest extant textual source, the printer’s manuscript: “& it came to pass that Shiz did persue Coriantumr eastward even to the borders by the sea shore”. But in Ether 14:12–14, the identity of “the sea shore & it came to pass” on two adjacent lines in 𝓞 led Oliver, I would propose, to accidentally miscopy the by in verse 12 as upon.
Summary: Emend in Ether 14:12 the preposition upon to the expected by (thus “to the borders by the seashore”); the preposition upon appears to be an error from the phrase “upon the seashore” found in the immediately following line of text in verse 13 (in the printer’s manuscript and presumably the same in the original manuscript).