I know you aren't supposed to mess with those adjustments and I have only done it a couple times. Both times the situation was as is yours; couldn't make it worse.
The one that I had the best results was on a Ferrari (someone had done as you are saying and really screwed it up). It has about the same injection. The biggest difference was that the car has individual exhaust taps so that exhaust gas readings can be had for each cylinder.
Your method for adjusting might not be sensitive enough and might need some refining. First I wouldn't disconnect the frequency valve (if that is what was meant by the "Lambda valve"). Doing this alters the differential pressures in a way that wouldn't happen in actual mixture control. What you should do is disconnect the Oxygen Sensor so that the system will quit trying to correct your mixture corrections.
Since the motor actually runs better with excessively rich mixtures. You are going to be left with a huge mixture window to shoot for. I would suggest a couple other techniques. Unfortunately they require exotic special tools; the best of which virtually no one has anymore.
The best method would be to find someone with a differential flow meter. This tool is pictured in the MB shop manual but I don't think any MB dealer has one. It was a required tool of all Bosch Service Centers so you might find one there. We of course have one and that is the way I set the other couple of distributors I have reset. This tool hooks to the injectors sort of like a milking machine works on a cow. The fuel pump is energized and the airflow meter plate is deflected by a special screw feature that holds the deflection at three different flow rates 10cc/minute, 35cc/min, and 60cc/min. These represent approx idle, midrange and full throttle conditions. Each injector and fuel distributor circuit is measured and must be within 10% different at each flow rate.
Doing this set-up using this tool indentifies the ability to correct. Often the differential between cylinder to cylinder is due to pressure losses across the stainless diagphram. In these cases adjustment won't work as the leaking affects the situation more at low flow rates. Thus if you alter idle, you may really screw the other settings. This will become apparent when testing the other rates.
If this tool isn't available, your scheme might work if you can use an exhaust gas analyser. Unfortunately you will not be able to test the results under load (without a Dyno - got one of those too) and as such you won't know how you have affected this condition till another sign comes up. Luckily that motor can't hardly be hurt by lean mixtures in the short range unless you race it. If that sparkplug gets burned up - lean, I would get that fuel dist. replaced.
Bosch Master, ASE Master, L1
33 years MB technician