What makes you think that it cannot be proven in court that Frank knows his statements are false? Thus far, the courts believe Mignini.
It should be a very straight forward task of demonstrating proof. Either there are drug dealers that are connected with Mignini, or there are not. Common sense suggests that it is not reasonable to assume that prosecutors have ties to drug dealers, so ... let's see the proof. In fact, I have to wonder why the drug dealers aren't coming to Frank's rescue at this moment ... couldn't they also help Knox by demonstrating that the prosecutor is a druggie? Doesn't everyone want to help Knox?
Otto, which is it? That he's buddies with drug dealers, using drug dealers as pawns in his trials, or using drugs himself? You make a lot of concrete assumptions about the nature of the allegations without even knowing what they are or if there is truth to them. If you do know, please fill us in. I think knowing what the allegations are first would be a good starting point for this discussion. But you're putting the cart before the horse and just assuming that Sfarzo has lied about something and is therefore guilty. Right now all we know is that Sfarzo has claimed something and Mignini disagrees. Until we know what exactly is going on it's premature to say "Frank is lying so of course Mignini is suing him". If he has lied, then I agree that a lawsuit is probably justified, but as we know Mignini already has an abuse of power conviction and an ongoing investigation by the CPJ which doesn't make his claims look good. Sfarzo, as far as I know, has never lied about anything and has apparently had the crap beaten out of him for voicing his opinion about Mignini - so I feel he is the more honest of the two.