This assumes that both lawyers were of equal competence. The authors does not think so (other members of the jury didn't think so).
In the end, won't the jury have to interpret "case law"? How can you be presented something without interpreting it.
Yes you interpret it, as it is described and presented (in lay terms) by both the defense and prosecution, as filtered by an impartial judge.
And incompetent representation is grounds for an appeal. I'm not sure what exactly that has to do with the fact that John Doe off the street is probably not equipped to read primary source material on legal precedent and interpret it correct on his own.
You are not, as someone with no legal training, no background in law or litigation, and no experience in the courtroom outside of being on a jury, supposed to start reading decades old case law and attempting to interpret it.
Lawyers are not paid $300 an hour because they've come up with a sweet gig, it's because the average person (yes, even the average software developer) does not have the context or experience to understand the finer points of the laws in question.