I went another way in the new book. I decided that the exploding rocks were an accidental byproduct of the terraforming process that maintained idyllic, garden-like conditions across the entire planet, and that the poison plants were not toxic to the Vaalians, but only to intruders.
And, yeah, I assumed the Vaalians were "feeding" Vaal with fruits, vegetables, and other organic matter . . . .which I rationalized as being a sustainable energy source that could be safely provided by even a primitive people without any technological skills.