Had the case gone to trial, Sarah Ivens would been put on the witness stand as the prosecuting witness or "prosecutrix," as the young girls were called in these cases of lewd and lascivious acts. She wasn't totally blind. Her daughter, Marianne Underwood Beveridge, stated on this discussion list that, though she was "visually impaired, having very little sight," she could see "at close range." Sarah would have been allowed to try to identify the defendant in the courtroom at a range comparable to her original encounter with him. Recall that the allegation was that she and Bix were together in a garage and in an automobile whose door Bix closed. Of course, her identification would have been subject to impeachment on cross-examination for her sight-impairment.
Even if Sarah could not easily identify her assailant, or if her identification of Bix had been strongly impeached on cross-examination, the county attorney could have elicited testimony from James Duncan and Mahlon Bailey that they saw Bix take Sarah into the garage and heard her holler and saw her leave the garage for home. The jury would have been allowed to draw the inference that Bix was at the scene of the alleged crime.
The research some of us have done on this case has not--so far as I know--turned up any reason to believe that Duncan and Bailey were biased against or hostile towards Bix. (I discount Roba's speculations about anti-German sentiment.) But if a factual basis for bias did exist, Bix's attorney could have used it on cross-examination to impeach Duncan's and Bailey's testimony.