Another trick I've used for the past 30 years is to cobble something up so the front mounting points of the seat are about an inch higher. This will support the back of the thighs, as you mentioned. This should take some pressure off the butt. Since most cars now have power seats adjustable in all kinds of positions, I haven't had to do this in a lot of cars, including my MB.

It looks like the next thing on your list is to find a mechanic who can adjust your Bowden cable. I've seen a DIY writeup of this procedure a few years back somewhere in this forum. I've never done it myself. Try searching the srchives using the keywords "bowden cable adjust".

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95 E320 Cabriolet, 169K