A speaker of modern English might wonder if the bare expression “in the midst” isn’t a mistake for “in the midst of X” (where X refers to people). Nonetheless, when X refers to a lot of people, the postmodifying prepositional “of X” is frequently left unexpressed in the Book of Mormon:
‘in the midst [of a crowd]’ (four times)
‘in the midst [of fallen soldiers]’ (two times)
We find eight instances in the King James New Testament that refer to a person being “in the midst” of a group of people, such as:
Thus there is nothing wrong with the expression “in the midst” in the Book of Mormon text. Most times, it turns out, the “of X” is expressed (13 times), as in 3 Nephi 27:2: “and Jesus came and stood in the midst of them”.
Summary: Maintain the bare expression “in the midst” without any postmodification whenever it occurs in the earliest Book of Mormon text (here in 3 Nephi 17:12 and in five other passages).