> True, but those experiments are inevitably restricted to the aspects of consciousness available for indirect observation and comparison.

As noted before, all observations are indirect. That has not been the unique challenge for studying consciousness.

The unique challenge has been an inability to create constructive or destructive experiments.

We didn't know how to construct consciousness candidates or precursors, of varying kinds to study. We are getting close to that (i.e. self-awareness as a functional quality is certainly a precursor.)

And we have not been able to create controlled demolition of an consciousness candidates, due to the morals of human experiments. (But our artificial minds will be alterable and destructible at will, to test hypothesis.)