Why would an Accessory gear box failure cause an N2 seizure in the first place?
I dont think it would,just an engine failure(flame out as you said) as the fuel pump assemply is driven by the AGB.
The AGB drives the Engine Hydraulic Pump, so no pressure.
Why N2 would indicate 0 immediately?I am not 100 perc sure but the N2 sensor is connected to the front of the AGB,so if the AGB failed,the sensor would not get any info about the N2 rotors speed.