After contacting Unity support I am getting response about described issues previously posted. The native plugin renders geometry at end of frame when scene is completed. So it is not good idea to use it for rendering trees and grass. Also it is why trees and grass are rendered over transparency objects. Shadows are not available through native plugin interface. So the solution is to use integrated rendering pipelite for rendering trees and grass. It means that I stopped working on implementation through native plugin and I will rewrite rendering part to Graphic.DrawMesh function.