For the shadow acne, try culling the front faces when rendering your shadow map (if you're not already. I didn't see it in your code, but I'm not terribly familiar with directx. But, I imagine directx lets you front/back cull faces). Then go back to back-face culling when rendering the scene.
yes do that, render backfaces to your shadowmap
think about how big a texel from the shadowmap is if it's projected onto your terrain, clearly the area each texel has to cover is bigger if the light is low above the ground and producing very long shadows. biasing can correct this to a certain extent but if your shadowmap resolution is too small that won't help either. a simple solution would be to just increase the shadowmap resolution, and/or limit the possible light angle.
i think a proper solution would be to use a cascaded shadowmapping technique