Have my doubts about your theory of constantly and rapidly sinking pellets. It is obvious that heavier objects are left behind when soil erodes from wind or water. Time for a test in sandy, silty, and clayey agricultural soils under various cultivation techniques.
And I'd guess that both lead and steel shot would pass through the digestive system of coyotes with no loss of mass, rendering the consumption of dove carcasses mostly harmless. I have never heard of a wild fox or coyote that was shown to have died from lead poisoning.