I haven’t followed the legal contours of this case very closely so perhaps these are not interesting questions.

Who would go to jail if _no_ driver was in the car?

From what I recall, the vehicle was a Volvo and its pedestrian detection and emergency braking system was defeated by the test equipment. Is there any culpability there?

Or what about the person who planned and initiated the driving route that night, in the dark through a populated area. Are they responsible to a degree?

It doesn’t sound like the backup “driver” was engaging in engineering or testing activities, and certainly not driving, but instead was paid to be doing what a passenger might do if they thought someone else was driving.

What the driver is legally responsible for may be different from what they believed they were doing. At least, that would me my defense in the situation.