Rob,
Burning the midnight oil again I see.
Excellent write up on the workings of a flash unit. You are the electronics boffin, so would not disagree with anything you have said.
Your mention of Cadmium Sulfide photocells (CsS) in the trigger is interesting. My trigger of unknown origin dates from the mid 80's so I would not expect the light sensor to be very sensitive. My tests involed every permutation I could think of. Pop up - big flash - distance from trigger, all to no avail
Camera light meter cells went through the route from CDS to SPD with its faster response and greater sensitivity to light, so one might have expected cells used in flash triggers to have gone a similar route. My thought was that if the pre flash is not as bright as the main flash then possibly we could make use of the inverse square law to limit the light intensity the sensor recieves. Hence the black flags.
Graphic attatched.
May work, may not.
Don