This seems to be building up to the important point that there are two substantial, and in many ways separate, functions to test aircrew training FTE / FTO / FTN...)
(1) Protecting them - including equipment and aircrew training, and
(2) Protecting the aircraft and trial - which includes their conduct, knowledge of the requirements, their ability to make a useful contribution to the trial and if required to the operation of the aircraft.
(1) Is reasonably easily dealt with by a few days aeromed type course or by using supernumary qualified pilots, it's (2) that is incredibly difficult to define and likely to differ with every flight test task.
Which surely has got to come down to the judgements and planning of the senior people in the flight test organisation, but doesn't lend itself to absolutes.
G