Have read this post and only saw one small mention by 'mctwin2kman' of the "Deceleration throttle micro-switch" which sits at the end of the throttle linkage. With the accelerator pedal up the top arm of the throttle linkage rests on the little button switch and needs to depress it sufficiently for it to 'click'. Have you checked/discounted this?

I only mention it as I had a similar high idle problem last week (1500 RPM in 'P' when engines at working temp) after getting the car back from the garage - was having the crankshaft position sensor replaced. As the mechanic duly went on holiday I did a bit of reading up to try to fix the problem myself. From some other posts I read about the significance of this little switch, and sure enough my linkage had somehow been altered so the switch no longer clicked. All I had to do was turn a little black plastic adjustment screw on the linkage a half turn, which slackened off the cable sufficiently for the spring to pull the arm more forcefully onto the switch. Immediately the switch clicked the idle revs dropped to 700'ish!

Might not be the solution for you but just thought I'd throw it in in case other readers are suffering due to something so trivial.

The reason I didn't mention the throttle linkage microswitch is that it doesn't apply in this case. The original poster has a 560 SEL and these cars don't use this type of switch. Instead they rely on the position of the TVS for "thrust cut-off".

T.C.