Here we have, based on the printer’s manuscript, one case of “on the north” and one of “in the north”. The original manuscript is not extant for either of these prepositional phrases, but usage suggests that the case of “in the north” could be an error for “on the north”. Elsewhere in the text, we find evidence for only the preposition on before a cardinal direction immediately followed by a postmodifying by-phrase:
In fact, in the nearby example from Alma 50:13, the on was accidentally changed to in in the 1840 edition (see the discussion under that passage). There is also evidence that Oliver Cowdery sometimes mixed up on and in, as in the following two examples:
For two other passages where Oliver may have mixed up these two prepositions, see the discussion under Alma 56:1 and 3 Nephi 8:5.
All of this evidence supports the possible emendation of “in the north” to “on the north” in Alma 50:15. Nonetheless, there is nothing particularly difficult about referring to the city of Lehi as being “in the north by the borders of the seashore”. So in spite of the internal evidence in favor of “on the north”, the critical text will retain the earliest extant reading, “in the north by the borders of the seashore”.
Summary: Maintain in Alma 50:15 the preposition in in the phrase “in the north by the borders of the seashore”, the reading of 𝓟 (the earliest extant source) and all the printed editions; nonetheless, the possibility remains that the in here is an error for on.