Hya,
the issue on your old one was the motor on the rear of the needle giving up. These are either cross coils or stepper motors depending on the year built. The stepper motors have small plastic gears which can break very easily if a small piece of contamination got in. As for the cross coils they have silicon inside, if this dries out then the gears become jumpy and eventually stop moving. Sometimes if the speedo cluster is removed and not stood right the silicone runs out and the same result happens.
As for the mileage, yes it is 100% stored in the EEPROM data on the cluster, nothing at all to do with the ECU. Some dealerships cannot do this, they simpley dont have the equipment, but they should sort it out for you. It is a very costly thing to do at home as you need the original EEPROM data, canalyser software, an EEPROM writer, and the technical skills to use the software.
Alternatively if they have not sent the defective meter back under warranty/investigation, get them to swap the motors out. You have to be very careful in removing all the parts but it is standard practice in the assembly line in the factory to replace motors!!!!
Hope they do something for you!!