"The way it works is that it is attached to the pad and sits against the disk and wears down as the pad wears down, giving a reading of estimated mileage left until the pads are fully worn.."
Your car must be different from mine and all other BMW brake systems I have worked on. The wear sensor sits on the pad itself and will not activate until the pad is worn to the point of rotor to sensor contact at which point it closes the circuit and activates the pad indicator. Once the sensor is replaced the system should be reset within two driving cycles. Typically it resets when you power the car up.
The brake pad reading on Idrive is purely an estimate of your pad wear based on the data collected by the car it does not measure wear on the pads it is an estimate...