Hi - you are right that we cannot prescribe any medical treatment on this forum (and nor should you listen to any!) But, we can offer a few suggestions that you can discuss with your health care professional and see if it makes any sense for you.
First, for the readers who are not familiar with spondylolisthesis, this is a condition where a vertebra slips off the bone below it, usually moving forward. There are two varieties: the congenital one (which you say you have) which usually happens in children due to some birth defect, where the L5 vertebra slips forward of the sacrum, and the adult version, where it is more often the L4 vertebra that slips forward on the L5, usually due to arthritis or some degeneration of the bones. Sometimes the condition may arise due to trauma to the area (an accident, perhaps, or as a result of over training in gymnastics.) [For a broader introduction, see Wikipedia's article:
http://en.wikipedia.org/wiki/Spondylolisthesis. They have a nice X-ray of a lower back with this condition.]
It would be great if some readers who also have spondylolisthesis could share their experience with yoga. I know that general treatment of milder cases include strengthening and stretching the region. Yoga is perfect for that. Yang styles of yoga will strengthen the muscles in the lower back, creating more support for the spine and sacrum. Poses like Crocodile (which is a push up but on elbows, the knees can be on the floor, but don't sag your butt!) and Plank pose (full push up) may help to strengthen this area, without having to do a backbend. Forward bends will help stretch out the lower back.
You ask about Seal pose: generally extension of the back is not a good idea for spondylolisthesis. Are you sure you want to go there? Even twists, while doing a forward bend, are not recommended. Often the condition is accompanied by, and made worse by, tight hamstrings. So, forward folds could be great: Caterpillar and later on, Dangling, may work well for you.
If you do want to try backbends, and your health care professional is onside with the idea, you may want to start with a gentle one first, like Sphinx. I wouldn't recommend starting with Seal, as that is the deepest backbend. After Sphinx, try out Saddle, before going to Seal.
Here is a case report from a Yoga Therapist who worked with a patient who has spondylolisthesis.
http://www.yogatherapycenter.org/articl ... caster.pdf
Good luck! Let us know how you progress.
Bernie