First thoughts are that the stepper-motor (which controls the idle speed) is faulty. That would explain both the high idle at low engine temps and the stalling and stumbling as you roll to a stop. This component is really just a small electric motor and shouldn't be too expensive to replace.

But have the car scanned first to make sure there are no other obvious causes for this behaviour. A faulty engine temperature sensor or some other small component could also produce these symptoms.

It's an electrical or electronic fault that's causing your car to go into the default limp home mode. It's not possible to say exactly what is wrong, do that you need to take the car to a mechanic and have them diagnose the problem on their equipment.

No. It ran perfectly for five or six years before giving trouble, so I don't think you can blame Holden. Has the coolant been changed in the time since it left the factory, has it been overheated at any time, they're questions I would want answered before trying to find someone to blame.