Here the earliest text (as found in 𝓞) reads “as the Lamanites had passed the hill Riplah and came into the valley and began to cross the river Sidon”. In his editing for the 1837 edition, Joseph Smith emended came to come, which has been the reading in the printed editions until the 1920 LDS edition restored the original came to the LDS text. Here one can, in theory at least, interpret the original came as either the simple past-tense form (“and [they] came into the valley”) or the past participial form came (“and [they had] came into the valley”). Although the latter usage is nonstandard, the original text had perfective verb phrases like “had came” (see the discussion under 1 Nephi 5:1, 4). In fact, one could interpret the following instance of began as actually the past participle for the verb begin (that is, “and [they had] began to cross the river Sidon”, equivalent to “and [they had] begun to cross the river Sidon”). There is also evidence in the text for “had began” (see the discussion under Jacob 2:12).
In this passage, the critical text will restore the original came and maintain the began, but without textually resolving whether came and began are simple past-tense forms or past participles (or a past participle followed by a simple past-tense form, as in Joseph Smith’s editing for the 1837 edition). Any of these interpretations is theoretically possible. For some discussion, see the analysis under 1 Nephi 1:14 of “when my father had read and saw many great and marvelous things”. For a specific example of “and began” that may stand for “and [had] began”, see the discussion under Jacob 5:17 for “and it had sprang forth and began to bear fruit”. Also see the example in Alma 42:5 of “if Adam had put forth his hand immediately and partook of the tree of life”.
Summary: Restore in Alma 43:35 the original came, despite its possible ambiguity: “as the Lamanites had passed the hill Riplah and came into the valley and began to cross the river Sidon”.