1989 300E rough idle fixed ?

[benzdriver737]

This is a cry for help.
I've been trying to fix this rough idle for sometime now, and I've read many posts on the topic.
First thing I replaced were the two rubber hoses that go to the ICV, this helped.
New air filter, new plugs, (NGK regapped to 040 as per this forum) wires, cap, rotor... again this helped.
Oil and filter changed at regular interval (10,000 kms).
Buzzing noise from fuel pump fixed by replacing pump, along with fuel filter.
Replaced OVP relay and fuel pump relay.
New EHA along with eighth of a turn of screw, this helped.
New throttle cable. Lubricated throttle linkages. Adjusted bowden cable.
New O2 sensor.
New engine mounts. New coil. New voltage regulator.

The fuel injectors were replaced (supposedly) about 18 months ago.
I say (supposedly) because I'm not 100 % sure if the mechanic actually did replace them. (He was a crook and went broke because he lied to customers)
Maybe the injectors are old (how can you tell)
All along I've tried to keep the mixture reading at approx 50% as per duty cycle readout at x11 plug. (non California model)

Car starts instantly, drives perfectly, but till tonight, had a rough idle.
(along with a strong smell of exhaust fumes) It was not possible to drive with any of the windows down. The exhaust smell was too strong. Also the exhaust sound was a deep rumble, and because it was rough, it almost gave the impression that the motor was not running on all 6, not a smooth continuous sound.
Tonight, I pulled the ICV and cleaned it again (just to be sure) and then leaned the mixture screw down to 18% as per duty cycle readout. This had the effect of smoothing out the idle to near perfect and reducing the smell of exhaust to almost non existence. (rumble sound gone from exhaust now) sounds smooth (as it should.)
Took it for a drive, went like a charm... and lately, I've been watching the 'economy' guage and also watching the amount of fuel consumed. Tonight the 'economy' guage was hardly in the red.
Now I know 18% is lean, but the idle is absolutely perfect (well nearly).

So what does this mean.?
Does this point to something else not working.
18% duty cycle readout = smooth idle at the moment.
50% duty cycle readout = rough idle at the moment.

Another mechanic (a fairly reliable one, I'm led to believe) told me about twelve months ago that the fuel distributor was faulty and needed replacing.

Would a faulty fuel distributor display these symptoms.?

Car accelerates smoothly, no hesitation or misses, does not stall, plenty of power.
As I mentioned previously, starts instantly, hot or cold.
I apologise if I've missed a post somewhere that covers this.
Thoughts and opinions most welcome.