I was doing some Googling about the EAC last night and I came across
this article that insinuated that, while the plants of the EAC were fully domesticated, the strategy in which they were utilized wasn’t really that of fully sedentary agriculture, instead it was horticulture that was used as a supplement to a highly successful semi-nomadic foraging lifestyle focuses on the hickory nut. To wit, apparently the hickory trees would cyclically produce periods of plenty and then periods of few nuts in a strategy to cause boom-bust cycles in populations of nucivorous animals such as squirrels. During these lean times, the foraging strategy of the natives would switch to walnuts, which have some inherent disadvantages relative to hickory nuts. The seed crops of the EAC also grew well in the places that walnuts grew, which led to the natives gathering them along with the nuts, and eventually intentionally planting them, leading to their domestication.
This potentially helps explain why the EAC crops mostly disappeared when the Three Sisters arrived: it wasn’t just new crops that were coming in, but an entirely different way of life, that of sedentary agriculture.
In light of that, what I would propose would be not to focus on changes in rainfall to create the scarcity necessary for cultivation to intensify, but instead I would envision a scenario where at first, preferably the earlier the better, there be a relative abundance of hickory nuts during the fat years that drives up population density, leading to the domestication of the EAC crops to deal with the lack of nuts in the lean years, then, once the EAC is fully domestic, an epochal calamity like a blight to befall the hickory tree population and kneecap the nut-foraging strategy entirely for a period of say a thousand years or so, forcing the natives to subsist on the crops, and then around the same time for
@Revachah ’s mutant Osage orange to appear, making agriculture that much more productive.