If you do that it doesn't matter whether the background is segmented or not because Authorware doesn't have to reload the original bitmap to redraw anything. The foreground object was never in the background buffer so erasing it doesn't disturb the background pixels. It doesn't speed up the original display of the bitmap. In fact, it may slow it down a little. It's usually best to break a bitmap into horizontal segments.
Vertical segments can work, too.
In extreme cases you could break it into vertical and horizonal pieces. This is all pretty tweaky and is best left towards the end of a project when the screen layout isn't going to change. Also, see my answers to your specific questions below. They're different techniques and don't relate directly. We have our background at layer 0, and animations at layer 1.