The main thing to keep in mind is the distinction between argumentation and testimony. The whole point of logic is to develop techniques for evaluating the cogency of arguments independently of the arguer’s identity.
If the truth of a matter can not be established without a person’s testimoney, then the credibility and character come into play with respect to their integrity. The integrity of a testifier is essential in evaluation of the veracity of one’s testiment.
Excellent point.