The question hangs on the meaning of "witness". Misuse of the word by the media, possibly parliament (and now you?) gets us into trouble. If we constrain the word to mean a person who gives testimony in a court of law, perhaps your article need not have been written. Let me explain (to borrow a phrase I am certain I have seen somewhere).
The difficulty as I see it is that you have confused the structure and intentions of the committee that heard Kelly's "evidence" with those of a court of law. Had parliament intended the status of the committee to have any of the powers of a court, court rules would have applied. But this committee was not a court. It was a forum trying in its own way to produce a truth, and any consequences of its decision were for others to decide. One might argue about all sorts of aspects of its deliberations: how it handled "witnesses", whether its conclusions were politically motivated and so on. But the attraction of such committees is that they can be either inquisitorial or adversarial or both in their method of establishing their versions of truth.
There is a loose parallel between adjudication and a House of Commons committee when comparing them with the courts. Adjudication can be a fairly relaxed process as regards the quality and presentation of "witnesses" and "evidence". At the end of the process, if the parties are still dissatisfied, they can seek redress at the courts. Arbitration, however, which does demand the processes of much of the machinery of the law, restricts appeal to a court correspondingly, and its decisions are enforceable.
So, would it not be more helpful if we all – including the media – tried to use the terms "witness" and "evidence" more sparingly?
Malcolm Taylor, via email.