Sooooo.... I tried to get my hands on a working remote but was unable to find one. Then spend some more time doing research using both the web and the service manual - plus I had help a friend who was able to read an explain the circuit diagrams to me. We did some basic measuring and checking and it seems that (most likely) something's wrong with the large control board underneath the pickup assembly. Not entirely sure though. But in the end we were unable to narrow it down.
As rein-o had already mentioned it's a tricky machine and hard to repair - so I decided to tear it apart and sell the parts
. Got rid of it all in the meantime and am now focusing entirely on the two working machines I still own, a LD-S9 and a CLD-2950. The grip ring of the LD-S9 was in need of deep cleaning as some discs were slipping a little after starting playback plus the lubricant inside got sticky over the years so I cleaned everything and apllied new grease. And the loading belt inside the 2950 was worn out and had to be replaced. Fortunately I had this one completely checked and repaired in 2010 as the spindle motor was worn out and even got a second unit in storage now in case I need parts like pickup or gears etc.
Watched a few movies during the last couple of days and they both work just fine. Still amazed by the picture quality of the S9 by the way - aside from a little ghosting here and there its really clean and sharp even though I'm now using a 48" Panasonic Full HD Plasma. Suppose only the HiVision models could offer me an even better picture when it comes to LDs...