Great post.
Looking at the sketches it's clear that some very brilliant ideas about this car's profile and hindquarters got disemboweled on the way to the lathe.
The most compelling of all these sketches is here:
I believe there were two design briefs here: one for a totally unique car, and another (more realistic) call for an S-Klasse coupe.
There were great ideas here to do something pretty awesome, but instead we got something attractive and perfunctory. I can only guess that budget considerations tied this car down, since it's pretty low volume nook in Mercedes' range.
Looking at the above sketch I hope that some of these lines will work their way into the next-generation SL.