The second and joins two sentences rather than two predicates. Elsewhere in the Book of Mormon, there are 20 occurrences of the verb come followed by a one-word adverbial (namely, down, forth, up, or forward ) that is then conjoined by the conjunction and to a following clause or full predicate. And in each of these other instances, we have only the predicate following the and; that is, elsewhere a pronominal form of the subject is never repeated when the verb is come (thus “an angel came down and stood before me” in 1 Nephi 11:14). Consequently, the original wording in 1 Nephi 12:6 (where two sentences are joined rather than two predicates) seems out of place and may explain why Oliver Cowdery omitted the second he when he copied from 𝓞 into 𝓟.
When we consider other verbs besides come, we discover that there are a good many cases of a short sentence conjoined to a following sentence where the pronominal form of the subject occurs in both sentences, just as in the original manuscript for 1 Nephi 12:6. Here are some examples involving the subject pronouns he and they:
All of these examples have the conjunction and at the beginning of the initial sentence, just as in 1 Nephi 12:6. So the reading of the original manuscript in 1 Nephi 12:6 (“and he came down and he shewed himself unto them”) is quite acceptable.
Summary: Restore the subject pronoun he in the second conjoined sentence in “and he came down and he shewed himself unto them”.