re: The RIAA can't lose
"The defendant will get nothing out of fighting this case because any jury will almost certainly find that she broke the law beyond reasonable doubt."
You seem to be forgetting that, at least in theory, the laws are made by the people for the general good of the people. If enough people don't think that the RIAA is acting legally or in the best interests of the majority of the people, then eventually the laws will change to reflect this. This may start with a jury sending the RIAA a very clear message that their behaviour is NOT acceptable.
"The second point is that the defendant had absolutely no legal right to share any of these files"
Whether that is true or not, the RIAA still needs to prove 'beyond reasonable doubt' (your words) that those files were 'shared' (whatever that means). Just assuming that people are guilty because your employer (the RIAA - it's so obvious) says so, is not the best advert for a judicial system.