The 1830 edition here changed the singular design to the plural designs, probably accidentally. Elsewhere the Book of Mormon text has both singular and plural forms for this noun. In nine cases, we have the singular design without textual variance; in six cases, we have the plural designs, also without any textual variation. One example is similar to the one here in Alma 10:17 in that it too refers to knowing of someone’s intent, and this example has the singular design:
Thus the critical text will restore in Alma 10:17 the singular design, the reading of the printer’s manuscript (the earliest extant source).
For one of the invariant cases of the singular design, there is a possibility that design is an error for the plural designs. For discussion of that example, see under Alma 43:9.
Summary: Restore in Alma 10:17 the reading of the printer’s manuscript (the earliest extant source for this passage), with its singular design: “now they knew not that Amulek could know of their design”; usage elsewhere in the text allows for either singular design or plural designs.