1. I think the trees are mixing the texture, or their .material fails to have the texture listed. Where are these objects from/downloaded from?
I have no idea about the grass. o-o Look at the above post.
2. I'm afraid there is no way to move a part of the mesh with out using a program like Blender, AC3D, etc.
3. Yep! As long as you give credit, I think you're good.