The counter argument to the BIB would be that the CSC can technically only rule on points of law, so they didn't reach different conclusions to Hellmann based on revisiting the same facts, but rather overturned his verdict on legal grounds. I guess Amanda and Raffaele could try to argue that the CSC overstepped its mandate by ruling on the facts, but even then, that might be a tricky argument to make because the case did go back to a lower court for reassessment. So with Hellmann annulled, Nencini was just confirming Massei - who did hear most of the witnesses - rather than overturning Hellmann. Plus, the ECHR does recognize that in exceptional circumstances witnesses might not be heard in person, and I guess being dead would probably qualify as one of those exceptional circumstances!
There's also the fact that Curatolo wasn't really decisive for the verdict one way or another. If anything was decisive then I think it would have to be the DNA evidence (not least because it was described that way by the CSC) and I wonder if that - along with Amanda's calunnia case being held alongside the murder trial - might be their strongest appeal point. The reversal of the burden of proof suggested in the "impossible" requirement to prove contamination (to quote Gill) seems like a massive issue to me, and definitely the kind of issue in which the ECHR might get involved.
The ECHR is not an appeal court. They are a court that evaluates whether or not a country which is a member of the Council of Europe has violated an applicant's Convention rights. (The other roles of the ECHR are not relevant for this case, so far.)
The CSC annulment of the Hellmann court verdict does not eliminate it from consideration by the ECHR. It routinely evalutes such verdicts with their reasonings, because the ECHR role in such cases as this is determine if there has been any violation of the right to a fair trial (Convention Article 6).
Hellmann essentially overturned Massei. The question before ECHR will be, were the Massei court, the Hellmann court, the CSC, and Nencini, and the next CSC decision each arbitrary or reasonable. The ECHR will review the CSC decisions from a practical viewpoint, and whether CSC decisions were on law or actually on evidence but disguised as points of law, is not so much relevant, as whether the ECHR considers the CSC decisions as compatible with a fair trial.
You are being somewhat legalistic in your interpretations of what ECHR does, and that includes your reference to Curatolo. Of course, the ECHR would not expect the Nencini court to rehear a deceased witness. The question is one of the interpretation by the court of the testimonial record of the witness. Was the Nencini (and CSC) interpretation of Curatolo's testimonies reasonable, in light of the prior testimonies and interpretations, including Hellmann.
Curatolo's evidence does not show that AK and RS committed a murder. In fact, Curatolo's testimony in the Hellmann court, showing he was not reliable on which day (Oct. 31 or Nov. 1) he allegedly saw AK and RS, showed that his testimony was unreliable. How did the Nencini court overcome this, and was the argument reasonable?
You are correct in describing the issue of the alleged DNA evidence as being critical to conviction, but so too may be the prosecution's multiple attacker theory and the findings of Guede's fast-track trial and appeal, and the use of Amanda's statements made without counsel. Did the Nencini court or CSC provide a reasoned argument for the defense being required to prove contamination, or is that a reversal of the burden of proof? How does Nencini counter the independent experts, Conti and Vecchiotti, in their view that the alleged DNA evidence was not reliable? Are the interjections from Guede's trial and appeal, or from any of his uncross-examined statements, a violation of the presumption of innocence and the right to confront a witness? What role did Amanda's statements play in the murder conviction; were they rightfully included or excluded, in light of Salduz v Turkey and all the case-law deriving from that judgment? And is there an Article 3 element (use of inhuman and degrading treatment to elicit statements) to be considered as well?
Again, the ECHR, if and when it reviews this case, which will depend on the final verdict. If there were to be a final acquittal, there may be no ECHR case for the murder. But if there is a final conviction, then the issue of conviction after acquittal will come up, but not in as automatic US-style double jeopardy issue, but as an evaluation of evidence and the use - including interpretation - of evidence. Conviction after acquittal raises concerns about arbitrary reasoning at ECHR, but does not result in an automatic finding of a violation of Article 6.
The question whether judicial reasoning was arbitrary or not in the opinion of the ECHR and its view of international judicial standards will arise.
