DO NOT go pulling the relay panel, in 35 years of owning many 928's and troubleshooting your particular problem, in my opinion its not there!
Your starter circuit is good, could still be a relay, I ASSUME you have the correct/new Fuel Injection relay 928.615.119.01 in XVI, no others will work!
I agree with Michael, so lets try this:
The air flow sensor is a simple piece of hardware, it send the air temp and the flap position to the ecu, thats all. The flap position is pin 7 on the diagram, air flow is pin 27. You will only get valid test readings if both the ecu plug and the afm are plugged in. green/red trace ( 2nd wire from top in your pictures below) to ground with as voltmeter should change as you move the flap, (ign on so the ecu is powered) you will not see any variation on green, blue green, or the other wire.
The following is a comprehensive test of the ecu/afm, with ignition off, disconnect the ecu plug, and check :
This also eliminates the idle shut down and full throttle switches:
1) Accelerator pedal released.
2) Accelerator pedal slightly depressed (throttle linkage play eliminated1
3) Accelerator pedal released.
4) Accelerator pedal depressed (until throttle open more than 300).
