If you think any competent lawyer couldn't make a case out of Joseph getting leniency for not pulling the trigger if his client had also not pulled the trigger, then you should re-adjust your self perception. "Mental gymnastics." Please.
Hell, if it's merely Joseph's testimony and he doesn't face charges that's going to be a sticky subject at the trials which I'd expect a competent lawyer to attack.
"The state's evidence against my client is based on the word of an individual who didn't come forward until he was publicly identified. Not only that, but he has been given immunity. This man is a privileged member of society, etc."