Very nice images!
For the door handle, the reason might be the following: AmbientOcclusion uses only the front material when rendering, so it might be picking up the wrong material when rendering. Can you try applying the same material on both the front and back faces? Another reason might be that there are some overlapping faces, causing some self-occlusion.
We are adding support for a background image very soon. In the meanwhile, can you try again with the dome, but at the same time reduce the "Occlusion Distance" paramenter, to something like 5-10?