If George is successful at the immunity hearing (highly doubtful, I know), do you think he would have a cause of action against Corey, et. al. for his arrest, etc.?
No. The immunity provision only give a money claim against anybody who brings a civil suit. One always has the option of filing formal ethics complaints and suing for wrongful prosecution, or a civil rights action for deprivation of rights under color of law, but getting money damages from the state is near impossible. Prosecutors are the second most immune species on the planet, judges being first, cops being third. Doesn't mean he doesn't have a case. I think it is a slam dunk to find Corey in an ethics violation.
FWIW, I think Zimmerman is going to get immunity, easily. The facts make this an easy case, not a hard one. I think Judge Lester is unimpressed with the politics, and even if he was, he knows that public sentiment has been created by a gross misrepresentation of the facts (adn law), by the press. I don't think he wants to jump in the pool with Corey.