Re: Why bother?
Judges decide issues of law, juries decide issues of fact. So e.g. if the law states that someone must do A, B, C to achieve crime Q but the prosecution presents only evidence of A and B — failing even to mention C — then the judge should decide that the jury cannot find a verdict on crime Q. Specifically what happens is that the judge phrases the questions the jury must answer. If the jury answers different questions then that has no effect.
Like if they came back and said "the defendant is not guilty... because Ted did it!"; if Ted weren't a defendant then that wouldn't mean he'd been found guilty of the crime.