1) Don't know if the DSC system uses the ABS sensors, but the first thing that comes to my mind is the ABS sensors cleaned/functioning properly? Some times it can be as simple as this.
2) The squeeky suspension: Not too sure about this, but again, first thing that comes to my mind are old struts/shocks/springs. But signs of old shocks is usually a rattling noise...just givin you some idea's is all. Do wait for a better answer on this one.
3) Can't help ya there, but you might have more luck at The Seven Series Register. Post on the E38 board, ATTN: Petetina, as he did some upgrades on his E38 NAV system just recently. He might be able to help you out. You will have to register for this site...tell them forzabimmer sent you...
http://www.the7seriesregister.co.uk/forums/index.php
4) ???
5) See if there were any recalls on this...you might be able to get them replaced for FREEEEE...(but then again, that might be wishful thinkin)
6) The switches can become gummed up or worn out. You might be able to take the switch out and clean it with some contact cleaner. Dry it off completely and let air dry for a few minutes before reattaching and testing. Is it just one function or does the whole seat not move with any function? Check fuses to see if they aren't blown.