Russo, Jim is a much better technician than I am, so his response makes far more sense than anything I might write. But basically my muddle through it see what works approach works out to about what he described. I scan my "challenging" slides usually at default settings, and then use Photoshop to try and fix the problems.
The shadow/highlight tool is at its best when the image is reasonably well lit but you want to bring out some shadow detail (usually the rods) or burn in an overexposed sky. If it is really backlit the "fix" usually results in a "painterly" result that resembles a Ted Rose painting. In addition to the shadow/highlight tool you may have to adjust the resulting image with about every other tool in the tool box...it can be a real trial and error effort.
But backlit is backlit, so it is tough to make it look otherwise. And a lot of my favorite shots are just backlit.
The "halo" effect is very common, and gets worse if your scanner lens starts picking up dust. I had to throw out my old Polaroid scanner because the lens got dirty and there was no practical way to clean it. The Nikon scanners appear to protect the lens better, but I cover mine with a custom dust cover. With regard to the halo I have found two fixes, one is to selectively increase the contrast in that area (usually by darkening the dark part), or in some extreme cases I have used the cloning tool to create a new sharp edge.
While Photoshop Elements is a great tool for the money, and all you need for most digital originals, for correcting scanned slides I think the full blown Photoshop has some much better color correction tools.
JBWX