The 1830 edition replaced the modal should with shall, probably under the influence of the nearly parallel phraseology earlier in this verse:
However, earlier in this passage (in the immediately preceding verse), we find should used with the phrase “fall into transgression”:
Note here that the occurrence of should (the first should in verse 21) extends to the following conjoined predicate (“if they should … fall into transgression”). In other words, we have more than one example in the original text of “ should fall into transgression” within the larger passage.
It is worth noting here that this 1830 error was not caught when the 1830 sheet (the 22nd signature) was proofed against 𝓞 (as discussed under Alma 42:31). For another instance where the 1830 typesetter replaced should with shall, see under 2 Nephi 25:19; for other cases where shall and should have been mixed up in the history of the text, see under modals in volume 3.
Summary: Restore in Alma 46:22 the original should at the end of the verse (“if we should fall into transgression”) while maintaining the shall earlier on in the verse (“if we shall fall into transgression”); in general, either reading is possible, so in each case we follow the earliest textual sources.