Q. Did they all just blackmail the mother to confess the murder?
A. Yes, and they did a mighty fine job of it, including the Amazing Unethical DA.
Q. Were they sure she had done it?
A. As far as I could see, the evidence -even disregarding questions of admissibility- was not clear-cut against her.
Q. WAITASECOND. So even after she confesses, it's possible that she didn't do it, and that she's only confessing to protect her daughter.
A. Yep. Hence my freakout.
Either this is an Evil "I just want to nab somebody" Bobby, or the writers just missed the honking plot gap, or I missed something. The possibilities are, you understand, ordered with increasing probability.
And now I'm weirdly intrigued by the concept of an AU Evil Bobby. Who then proceeds to have a passionate affair with Nicole, whom of course conceives the Antichrist.
ETA: Said Antichrist, of course, would grow up to be the "Bernardified Metrosexual Evilish Tim" I imagine every time I see the latest "version" of the Riddler.