Hi, Scandi, it's always a stimulating joy to read your posts. And, yes, MrsPC and twocuriouscats have, in my view, enlivened the case again. This is such a great community.
I agree that the woman provides a significant link. To me, that Shannan refused help from another woman indicates how frantic her probably drug-fueled mania had made her. I think that Sherre has been out of the media because her encounter (at least from what I have read) was over so quickly and, unlike the old man or the driver, she has not had a more complicated narrative to share. (In my view, the old man has presented slightly different stories at different times, not because he's lying, but because he's trying to extract meaning from his relatively brief encounter with a woman who he probably worries he could have saved.)
To me, Shannan's behavior is fully understandable within the context of her bipolarity, her cocaine and prescription drug abuse, and her job. For nearly forty years, I have lived on a block that was, until it was "cleaned up", a favorite shopping mart for johns. Joel Rifkind, I have read, picked up hookers on one corner, and Jody Foster trained for her Taxi Driver prostitute role on the other. I saw sex workers flip out, attack one another with fists and broken bottles, fall down steps (as Shannan apparently did), talk to themselves, and pass out next to my mailbox. One prostitute who lived in my building died of an overdose; she was a college graduate. Another woman, two blocks away, was murdered, then stuffed in a suitcase and abandoned in a vacant lot; this case, as far as I know, has still not been solved. Women I would see every day on my way home from work would one day just be gone. That a poor young woman in such a situation as Shannan was that morning would be imperiled when bolting in a relatively unpopulated area does not surprise me.