I'd lke to urge you to look at your fallacies here.
First, as a logical point, if truth is a good defence from an accusation of libel, this means that Knox has a possibility to defend herself, but that also implies that, exactly like in 1735, an accusation must moved against her in order to let her defend herself by asserting her "truth".
Second, there is a factual error in your statements. Mignini is not acting against here parents. It's a number of police officers who filed a defamation lawsuit against her parents, not Mignini.
Third, I point out your statement "I know she did because her account stands up to the scrutiny of the reasonable man/woman", because I notice that nobody among the pro-Knoxes accuses you to peddle hearsay as incontrovertible truths or of not providing scientific evidence. You assume a report as truth simply because it seems reasonable to you, and nobody accuses you to only bring unsubstantiated assertions.
Fourth: without questioning the point that Knox's report of being hit, if taken in isolation, itself is reasonable, I must point out there are, however a few other factual elements attached to it, that have their legal value. A factual element is that Ms. Knox did not sublit any legal complaint about being hit, and another fact is that her own lawyers publicly denied she was hit. This is a problem in Knox's defence, since you cannot take the first report in isolation from the other elements, because of the rules of the game. If the victim doesn't complain, that has a value like expressing the position that her rights were not violated and factually bars the allegation from being investigated.
Fifth: Mignini's is not a thuggish behaviour. There is no reason why he should be seen as a thug. He even has no initiative in the prosecution of Knox's parents, and would also have no interest in that (since he was not the defamed party). Thats police officers' initiative to protect their legitimate interest. Anyway it's something they have the right to do, in which Mignini had no power to initiate anything.