Interesting. I'm looking at doing a horsehair and spring assembly changeout on my '82 300D. Mostly its the spring assembly that leans to the outside - I presume due to "large man" syndrome, dropping on that side of the seat from some height to manage entry to the vehicle.
It's had the "large man" treatment enough to have popped a hole in the seat cover where the adjustment handle got crunched against the seat!
I doubt my ability to simply disassemble the seat and bend the springs somehow to get them "strong" again - so new items are probably in order.
How about a picture of the top of the seat cover so we can take a guess if its leather or MB-Tex?
Your seat looks like the way my '87 300D feels after riding in the '82 - like it doesn't have any padding left, and its leather in the '87.