Not a Lawyer , but here is my advice.
They are definitely trying to scam you.
1]Don’t pay , just go along with trial.
2]As far as I know , if someone commits a crime in juvenile age then no matter what his age will be at the trial he will be treated a juvenile. Few rare cases were there when they are treated as an adult but as you said he has few months before turning 18 , so don’t worry.
3]Nope , don’t plead guilty it will be a mistake. Keep those MRI documents safe and ask his friends who were playing them to testify if it’s required.