So here is my last post on this issue, solved.
If leaving all night the battery charger plugged+new battery was enough for GuruX to reboot the Instrument Clusters, the guy specialized in repairing Instrument Clusters had to over-heat it using a heat gun -on the back side- in order to rejuvenate it (a trick when some component are on their last leg?).
He told me it can work one day or one year, so I found & bought the exact 8Z092093Q (same 50€+37, also NL) and he could do the copy to the new IC. I also bring him the ECU to cancel immotronic (I can now use the plastic key going to the beach!). He took total 100€ and I didn't had to bring him the car.
I reinstall everything, and it works first time - so nothing else on the car was bad! However as clearing all the errors, I have too many errors with module 08 Klima to even have connection. This is new, can this cause trouble? The troubles I had?
As well as I have now on comfort module: 01330 - Calculateur central pour système confort (J393) 37-00 - Défectueux
But comfort module seems to perfectly works (as the air-conditioning) so I just have to ignore it I guess.