I'm not sure about right vs left controls, as I've never owned a V12.

The CEL only illuminates for emissions-related faults. It's possible to have component failures where the computers don't realize emissions are being affected. And, also possible to have the engine running terribly but never trigger a CEL because there's nothing faulty with emissions stuffs.

I don't have a 140 + M120 diagnostic document, but this one for the 140 + M119 will cover quite a few of the various systems except for the engine:
http://www.w124performance.com/docs/mb/diagnostics/DTC_List_W140_M119_1992-95.pdf

And, this factory DTC manual will cover what you need for the M120:
http://www.w124performance.com/docs/mb/diagnostics/DTC_Reference_Guide_Ed.B.pdf

If the wires to your MAF's are disintegrating (I assume you've read up on the biodegradeable insulation problem), fix that first.