Here we appear to have an example of subject-verb agreement that is determined by proximity rather than semantics, namely, the verb form for be takes the plural form were because the nearest noun, Nephites, is in the plural (“and now the design of the Nephites were to support their lands”). This kind of subject-verb disagreement based on proximity was fairly common in the original text. For discussion, see under 1 Nephi 13:23; for a more general discussion, see under subject-verb agreement in volume 3. Joseph Smith, in his editing for the 1837 edition, changed the plural were here in Alma 43:9 to the singular was.
Of course, one could argue that the original text actually read “and now the designs of the Nephites were to support their lands”—that is, designs instead of design. Note in particular that the two preceding verses have the plural designs:
But as explained under Alma 10:17, either singular design or plural designs is possible. Moreover, there is a strong tendency to use design rather than designs when there is more than one person (the statistics here are based on the earliest readings):
Thus the reference to “the design of the Nephites”, with its singular design, is expected. On the other hand, with only one person, the possibility of either design or designs is about the same:
Textually, there is only one example of variation in number for the word design(s), namely, in Alma 10:17 where the 1830 typesetter replaced design with designs (see the discussion under that passage). The scribes themselves never varied the number for design(s). Although it is possible that design here in Alma 43:9 is an error for designs, the earliest text supports design. The singular design is expected with a group of people, and proximity seems to have determined the choice of were after Nephites. The critical text will therefore maintain the earliest reading (“and now the design of the Nephites were to support their lands”), despite its nonstandard subject-verb agreement.
We should briefly note here the reading in the 1841 British edition where the phrase “the design of ” was accidentally omitted, giving the impossible “and now the Nephites was to support their lands”. In fact, the 1841 typesetter placed a comma after the Nephites. The source for his error of omission here is found in the previous line in his copytext, the 1837 edition; the 1841 typesetter momentarily let his eye stray up one line where the text correctly reads the Nephites followed by a comma:
This distraction caused him to omit “the design of ” and to add a comma after the Nephites in the next line. The following LDS edition (1849) restored the phrase “the design of ” and removed the extraneous comma.
Summary: Maintain in Alma 43:9 the reading of the earliest text, “the design of the Nephites were”; despite the subject-verb disagreement, the singular design is expected and the use of were in proximity to the plural Nephites is possible in the original text.