The original manuscript here reads may in the resultive that-clause, “that they may catch us in their snare”. Oliver Cowdery, when he copied the text from 𝓞 into 𝓟, replaced the may with might, probably unintentionally. His change to the conditional might could have been influenced by the conditional modal should in the preceding resultive that-clause (“that we should come against them”). The influence of the preceding should was more direct in the 1874 RLDS edition, which replaced the might with should. The 1908 RLDS edition restored the might of the printer’s manuscript. The critical text will restore the original may. See under Jacob 5:13 for a list of mixups between may and might.
Elsewhere in the original text, we have many examples of may and might in a subordinate clause where its preceding main clause has should, 16 with may and 41 with might. Although might is more frequent, may is clearly possible, as in the following example where may occurs three times:
In fact, the use of may in Alma 56:43 makes Helaman’s thinking seem less hypothetical, thus more accurately reflecting his thinking at the time of this battle.
Summary: In accord with the reading of the original manuscript, restore the modal may in Alma 56:43 (“that we should come against them / that they may catch us in their snare”).