I just found and joined this forum, and enjoyed the progress and pictures of the 500K. I built one of these (and the Fiat) in the mid 80's. An incredible amount of work and a lot of adjustments were needed to make things work. I had to dismantle and rebuild the rear swing axle about 5 or 6 times till it was satisfactory. As with you, I spent more time bending and adjusting the spokes than building the wheels. I did use the kit seats and upholstery, but backed the "leather" and foam padding with pieces of cut up bed sheet material, then hand stitched the "tuck and roll" effect. I then glued thick tan carpet thread at the seams to simulate piping. It came out pretty well, but I do like your resin seats as they are more prototypical. I also hand stitched the top, but wasn't satisfied with the top up appearance, so left it down all the time.
A few years ago, a friend of mine started building the Pocher Porsche 993, and gave up. I'm going to finish it for him at some time, and paint the body Arena Red as with his own 1:1 car, plus cut out and simulate an open sunroof. The fit of that kit is appalling, and you could tell that Pocher had limited funds at the end; the head and taillight reflectors are self adhesive stickers and look terrible. I'm going to attempt to scratchbuild some decent ones. They also didn't flock the interior carpeting, but supplied diecut individual self adhesive sections that were applied to the surfaces. Ugh. Wish he hadn't put then on. The seats and dash are molded in an awful lipstick pinkish red. I'm going to do grey seats w/ black dash per his real car, and flock the chassis base in black. The supplied seat belts and hardware are also poorly done, so will figure that one out. The engine detail is very poor, and hope I can dress it up a bit using some added detail. Nuts, the more I think about the amount of work I have to do, the less I want to do it!
Again, you're doing a magnificent job on the 500K, and am looking forward to pics of the finished product!