The amount of nitrogen produced by the beans which might be used by the corn is probably quite slight. In dietary terms beans are easy to store which is an important factor. Secondly they do provide both protein and carbohydrates. The protein was probably not so important to people who ate as much meat and fish as did the Mandan and Hidatsa of North Dakota. Beans, after threshing and winnowing, were usually stored in the same pits as the corn.
Wild sunflowers grow in the Upper Midwest and in many other places with sandy soils.
The seeds are oily and the plants are prolific. An Indian of five thousand years ago would simply pick the larger seeded plants and by planting those seeds hope that his or her crop would produce larger seeds than those produced the previous year. Using a basic process such as this produces results almost immediately. Sunflowers in the Amos Owen Garden were produced by using a similar technique with decorative sunflowers. 80 years ago I was able to produce these plants in just a few years.