Here we may have an example of a Hebraistic and separating a main clause from its preceding subordinate clause (“but had they awoke the Lamanites”) and an intervening parenthetical clause (“behold they were drunken”). In this instance, the subordinate clause is the conditional had- clause (where had precedes the subject). The and is not extant in 𝓞, but there is room for the ampersand in the lacuna. To be sure, 𝓟 has the ampersand. All the printed editions have maintained the and here, probably because one can include the parenthetical clause as part of the main clause—that is, the text can be read as “but had they awoke the Lamanites / behold they were drunken and the Nephites could have slain them”.
One can argue, however, that this and in Alma 55:18 could be the result of a scribal error on Oliver Cowdery’s part. Consider the following example in the next chapter where Oliver initially wrote an and after a conditional had-clause:
In this case, Oliver immediately erased the ampersand in 𝓞. Of course, there is a structural difference between these two cases of and: in Alma 55:18, there is an intervening parenthetical clause, “behold they were drunken” (at least under one interpretation), while in Alma 56:50 there is no intervening clause between the had-clause and the following main clause.
Ultimately, this instance of the extra and in Alma 55:18 is the only case in the earliest text of this particular Hebrew-like construction with inverted had-clauses (or where one can interpret the had-clause in this way). Nonetheless, there are quite a few instances with the much more frequent conditional construction, the if-clause (see, for instance, the discussion under 1 Nephi 17:50 and Helaman 12:13–21; also see the general discussion under hebraisms in volume 3). Moreover, many of these if-clauses have an intervening clause that more readily allows the occurrence of the Hebrew-like and before the main clause, as here in Alma 55:18. The critical text will therefore retain the and in this passage, especially since one can alternatively interpret the behold-clause as part of the main clause.
Summary: Maintain in Alma 55:18 the and that occurs before the clause “the Nephites could have slain them”.