You didn't mention any drive-ability issues before so I'm wondering if the mixture really was set incorrectly before the adjustments? Was changeover between petrol and gas smooth before? Is it now? How did the guys who made the adjustment ascertain that a mixture adjustment was necessary - Did they take it for a drive and switch between petrol / LPG and back etc both whilst driving and at idle and take their time in doing so? It is possible to adjust both the mutiplier (RC_Inj) and offset (Off_Inj) in Prins to give effectively the same mixture at idle but such an adjustment could affect the mid range and top end fueling, so the adjustments they made to multiplier and offset should have necessitated a drive in the car to at least confirm the adjustments made were correct.
A 1998 Merc CL420 may not have OBD2 and may have 0.8 to 1.6 Lambda probes. The former may mean the only way to check calibration is by using the switching fuels method whilst monitoring Pinj, time consuming and requiring a decent length drive in the car, the latter meaning the OpenLoop Prins settings should probably be disabled. I hope the guys didn't pay lip service to 'calibration issues' but this might be the case if they didn't take the car out for a good drive with laptop plugged in...
Another reason why I am pessimistic is because I have noted on many Mercs, especially those from around 1998 and especially those with 0.8 to 1.6 Lambdas, that they can be slow to steer mixture according to lambda readings particularly at idle and believe this is due to Merc knowing that a fluctuating mixture (as with usual quickly flipping lambda readings) doesn't provide as smooth an idle as a more constant mixture. If idle mixture goes rich the idle speed will increase until the idle valve decreases air flow and vice/versa, so I believe it is Mercs thinking (as a luxury marque) that they will beat the competition in idle smoothness with slower idle mixture adjustment.. The problem here regarding LPG tuning is that if the fuel trims have previously adapted over a long time to running with incorrect LPG calibration, any adjustment made to mixture is likely to give lambda lean or lambda rich readings until the fuel trims re-adapt (which due to the above can be longer than on most vehicles). This can give the impression to the installer that the current LPG calibration is correct and maybe even that there is some problem with the petrol system, when what they really need to be doing is running the vehicle on petrol for a fair time until the fuel trims are re-learned on petrol before attempting to adjust the LPG calibration.
Other things that can often go un-noticed on these engines but are a bit more noticeable running on LPG are cam sensor issues, old and slow to respond Lamdas (this being besides the slow to respond ECU's as mentioned above), broken Lambda heaters, long hoses between LPG injectors and manifold nozzles (particularly on these engines as the idle speed is only around 600rpm and the manifold design doesn't lend itself well to getting those large Keihin injector rails close to the manifold).
Many years ago I converted an E420 which had a slight issue of a very occasional single misfire at idle that occurred only slightly more often when running on LPG. I diagnosed a slow to respond Lambda and failing cam sensor and informed the owner. The owner wanted a second opinion so took the car to a couple of guys he knew and trusted, these were ex Merc main dealer mechanics who had set up by themselves with all the proper Merc diagnostic gear. These guys did their diagnostic but incorrectly blamed the LPG system (even though the problem also occurred on petrol) so the owner came back to me saying it was my fault.. I checked it over again and found the same Lambda and cam sensor issues, so he went back to the Merc mechanics, whom this time told him they couldn't do any further tests until the LPG system was completely disconnected as the LPG install interfered with their checks (a load of rubbish since when running on petrol the LPG system doesn't affect the petrol system at all). So against my better judgement but to keep the customer happy I disconnected the LPG system and back he went to the Merc mechanics who still couldn't find anything wrong. So the customer decided to go with my advice and had them change the Lambda probe and cam sensor anyway, and this fixed the slight issue completely, but only after all the messing about! The owner still has the car 10 years later and no longer goes to the Merc mechanics but I generally see him once a year (when he remembers) for his LPG service.
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
2 miles A1, 8 miles M62, http://www.Lpgc.co.uk