We examine matter-enhanced neutrino flavor transformation (ντ(μ)?νe) in the region above the neutrino sphere in type II supernovae. Our treatment explicitly includes contributions to the neutrino-propagation Hamiltonian from neutrino-neutrino forward scattering. A proper inclusion of these contributions shows that they have a completely negligible effect on the range of the νe-ντ(μ) vacuum mass-squared difference δm2, and vacuum mixing angle θ or equivalently sin22θ, required for enhanced supernova shock reheating. When neutrino background effects are included, we find that r-process nucleosynthesis from neutrino-heated supernova ejecta remains a sensitive probe of the mixing between a light νe and a ντ(μ) with a cosmologically significant mass. Neutrino-neutrino scattering contributions are found to have a generally small effect on the (δm2, sin22θ) parameter region probed by r-process nucleosynthesis. We point out that the nonlinear effects of the neutrino background extend the range of sensitivity of r-process nucleosynthesis to smaller values of δm2.