Rottenray .. re IP overspeed
You are correct re IP overspeed. Refer to ATSB preliminary investigation report AO-2010-089 figure A-2 on page 48. Note that it was the HP shaft (N3) that exhibited an overspeed condition. In addition, this was preceeded by an exceptional increase in N3 first order vibration. Those flight deck indications would undoubtedly have prompted a precautionary shutdown had there been sufficient time in which to respond.
Sensor Validation makes a valid point however!