1) collapse your group of separate meshes into a single mesh
2) corona scatter scatters based on the pivot point of your mesh, not it's bounding box, if you have large geometry this will happen. You can try shrinking the bounds of your scatter surface to accommodate for this
3) the only way this would happen is if you had a separate element with flipped normals on your scatter surface (or your pivot point on your objects isn't on the bottom of your mesh)