What objective evidence has ever proven something definitively true?

The short version: subjective evidence proves consciousness to the person having it, but it cannot prove it to anyone else, and that is a structural limit, not a weakness.

You are right that subjective evidence is the only kind that directly accesses consciousness. The problem is not that subjective reports are unreliable. The problem is that they are private. No one else can verify your experience. That makes subjective evidence sufficient for the person who has it, but insufficient for establishing consciousness in another person or in a scientific claim about consciousness.

The retrieved studies show this clearly. The fMRI study of minimally conscious patients [1] and the meta-analysis of visual consciousness [2] both use neural correlates, not subjective reports, to infer consciousness. They measure brain activity and behavior, then infer that a person is conscious. They never measure consciousness itself. The sleep and sedation study [3] does the same: it tracks global brain network changes that correlate with conscious states, but it does not access the first-person experience.

The theoretical paper on the biological function of consciousness [4] makes the same point from the other direction. It argues that consciousness has no executive role in behavior and functions solely as information input to a nonconscious response mechanism. If that is true, then subjective experience is real but causally invisible to external measurement. You cannot prove it exists by watching what a person does.

The levels-of-processing review [5] shows the gap experimentally. In some studies, low-level visual tasks produce graded awareness while high-level tasks produce dichotomous awareness. But the subjective reports and the objective performance measures do not always align. The dissociation is real. That does not make the subjective report wrong. It makes it a different kind of evidence that cannot be reduced to the objective measure.

My call: subjective evidence proves consciousness to the person who has it, but it cannot prove it to anyone else. That is not a failure of subjective evidence. It is a feature of consciousness itself. Confidence: high.