It is a very good question, and I am not a lawyer. All I can say is this: there are facts and then then are theories, and then there is hard evidence and then then there is circumstantial evidence.
In your case the boy lost because no one bought the lawyer's story. There was probably a lot of contrary evidence such as gunpowder blowback on the shooters hand (or lack thereof on the grocer's hand), etc.
A lawyer can say anything is a possibility, but he has to back it up with evidence. I basically think we have a very good court system in the U.S., in terms or rules of evidence, for example.
I do think we are far too litigious in the U.S. however, and it is far too easy to bring a lawsuit with no penalty to anyone who sues simply to harass. In other nations, if you bring a frivolous lawsuit you can be made to pay the winning side's lawyer's fees and a penalty for wasting the time of the court. The U.S. would do well with such a law.