The old saying is "innocent until proven guilty," although even if acquitted of a crime, a defendant is not "innocent," he is "not guilty." Regardless, the point is not whether Bix was guilty or not guilty in the eyes of the law -- certainly he was not guilty. The relevant questions for biographers are these: 1) did he do it? and 2) could he have done it?
Of course, neither question, at least with the information available to us today, is answerable. But that shouldn't deter investigation, because even unanswerable questions can yield insight. And in order to learn anything from such an investigation, in order to be anything other than Bix Beiderbecke's press agents and image consultants, we need to be open to all possible answers.