I have a slightly different take on this. I find it hard to believe that a DA with over 20 years of experience didn't suspect that she was a prostitute. In fact, I believe he was confident that the DNA on her was from the lacrosse players because he assumed they had paid her for sex.
I remember when I first heard the defense lawyers say that no sex whatsoever occurred that night...I was stunned! I bet Nifong fell out of his chair when he heard that.
I know my son and his friends would have made the big "EEE-UUU" sound had someone suggested they have sex with her or KR. I'd bet the nice young men from the Duke Lacross team wouldn't have considered it either, even if she had paid them.
Good point about why Nifong got so far out on a limb on the DNA.