The reading in the 1830 edition, with the preposition in, is a typo; the earliest textual source, the printer’s manuscript, reads into, which is what we expect in English. The 1837 edition restored the original into. Although there are no other occurrences of “come in(to) one’s heart” in the Book of Mormon, there are four in the King James Bible, all of which have into rather than in:
The critical text will maintain the preposition into in Helaman 13:3.
Summary: Retain the preposition into in Helaman 13:3 (“whatsoever things should come into his heart”), the reading of the earliest extant source (the printer’s manuscript).