One wonders here at the beginning of the relative clause (“in which Ammon and his brethren were cast by the servants of Limhi”) whether the preposition in might be an error for into, perhaps the result of the preceding in (“yea even in that same prison”). But then one might ask why the preceding in couldn’t also be a mistake for into. In other words, perhaps the original text here read “yea even into that same prison into which Ammon and his brethren were cast by the servants of Limhi”.
Except for this one place where in is twice used instead of the expected into, the text has only the preposition into for the phrase “cast in(to) prison(s)” (35 times); in fact, both earlier and later in this passage we have the preposition into: “and cast into prison … and after they had been cast into prison”. More generally, the verb cast prefers the preposition into (with 43 cases besides the 35 that refer to being cast into prison). Even so, there are four other cases where in occurs with the verb cast, although in these instances we have the phrasal verb cast in, where the in acts as an adverb rather than as a preposition. The first two cases use the expression “to cast in their voices” to refer to a vote on a political issue (in Mosiah 29:39 and Alma 2:6). But the two other cases are semantically closer to the cases of “cast into X” that we have been discussing:
These two instances of the phrasal verb cast in suggest that the preposition in can also be used with the verb cast. For each case of the phrase “cast in(to) X”, the critical text will follow the earliest text, thus retaining the two instances of in here in Helaman 5:21. The fact that the in is repeated argues for retaining it. Also note that in Alma 14:14 we have examples of both cast into and cast in (“which had been cast into the fire ... which were cast in with them”), just like here in Helaman 5:21–22.
Summary: Maintain in Helaman 5:21 the two instances of the preposition in for the phrase “cast in(to) prison”, even though elsewhere the text consistently uses the preposition into for this phrase; the repetition of the in in this passage (“yea even in that same prison in which Ammon and his brethren were cast by the servants of Limhi”) as well as the existence of the semantically related phrasal verb cast in argues that the preposition in is possible for “cast in(to) prison”.