Both the 1830 edition and the printer’s manuscript have the modal shall in this lest-clause. The 1837 edition changed the shall to should, probably accidentally. A similar example of shall being replaced with should in a lest-clause occurred later in the text, in the 1888 LDS edition:
Either reading is theoretically possible here in Helaman 15:9 and in Mormon 8:17, although the modal verb should is much more frequent in lest-clauses than shall (which probably explains the tendency to replace shall with should ). For the overall text, we get the following statistics for the use of modals in lest-clauses:
should 48 times
NULL (no modal) 21 times
shall 15 times
could 2 times
would 2 times
might 1 time
In each case, we select the modal found in the earliest text. The original use of shall in the lest- clause here in Helaman 15:9 is perfectly acceptable and will be restored in the critical text. For a comparison of the modals used in a similar conditional clause (namely, in the that-clause that complements the verb phrase “will/wilt ... suffer”), see under Alma 56:46.
Summary: Restore the original modal shall in Helaman 15:9: “lest by any means they shall sin” (the reading in 𝓟 and the 1830 edition, both firsthand copies of 𝓞).