Oh, and it's worth mentioning -- in the case I just described of an extorted plea, the prosecutor would be guilty of extortion even if the defendant was actually guilty.

In any case, the point is that the prosecutor is always vulnerable to being prosecuted for actual criminal conduct; and if criminal conduct is proven in court, they should lose the immunity shield against civil suit.

I could also go along with Degsme's suggestion that even a formal reprimand by the bar might be an acceptable indication that the prosecutor has transgressed the standards they are required to uphold in order to merit immunity.