...I'd still be curious how to remove leather from the seats, just in case I wanted to replace cushion pads. What's the trick to get it off?
I've replaced the skin on the bottom of my SL500, it's kinda difficult to explain, but basically, there is a metal channel all around the perimeter of the bottom of the seat. Skins have fiber strips sown in that tucks in those metal channels. To remove, the seat bottom should be in upside down position as you have in the picture, apply firm and even pressure (knee) to the middle of the metal frame, that relieves the tension of the skin and carefully, start pulling the fiber strip from the channel carefully, starting at the back.
Not sure if this helps, but here is someone's blog (early model, but the principle is the same), you can see the fiber strip and the channel fittings R129 500sl Restoration: Reupholstering the 500SL