Well, at Valenti they have changed both the lambda sensors, so I guess they will be fine.If the ghibli runs in safe mode, you will have a low boost, but quite a nice amount of pressure (0,7 bar if I remember well). So the car should not have any problem to ride on a hill and you should be able able to drive it almost without difficulties (with 0,7bar, you should have at least 200-220 hp I presume).
I'm not sure there are 2 different secure modes; in fact, I don't think so, only one secure mode (with low boost allowed) but no limp mode.
If the CEL light is ON even without firing the engine, the ecu doesn't receive the signal from one of the sensor (short circuit or defect in the wiring, or missing sensor -do you still have lambda sensor?).
More about the cause of the CEL here, I hope it will help you. http://www.maserati-alfieri.co.uk/alfieri06uuuuuu.htm
Did you try to disconnect the battery (24-48h) to re-initialise the ECU? But reading your description, I think the problem is elsewhere...
170 euro is an awful lot for 6km, I paid about 125 from my home to Valenti (50km, I live not far from "Le Roeulx"). For 170 euro, you can replace almost all the sensors of your ghibli.
We'll have the opportunity to meet, don't worry, Belgium is a small country.
SD1 doesn't seem very complicated; would you mind to share the diagram?
If Valenti is correct and there is a problem with my (left side) knock sensor signal, and it has to be changed I am not sure if I could do that myself (or trust myself doing this properly), as they are located kinda difficult, if I am not mistaken.
But I do have a knock sensor at home. But when I read your link (Enrico's page), I read it as if the CEL light would not light up before firing up the engine... Ok... Ok... AG25640 needed )))
Interesting to read that I should be able to drive up the hill... Currently I can drive up the Hill when I have some speed, but not from standing still. So ok, guess I will have to take a close look at the tubes to and from the turbos.
I think I did disconnect the battery long enough, but sure good idea, will do that again... won't harm and you never know she might fix herself ;-)
And sure happy to share the AG25640 diagram (see below), I found them somewhere on the internet. Hope you and others will find it useful.