Not only am I being a bit sweeping and vague, I am also changing the goal posts! (Not too much, I hope.) In other words, you are placing too much weight on one element of unfairness, when in fact ECHR evaluates over a range of elements of unfairness. I may have unintentionally mislead readers by not stressing that it is the question of the over-all fairness of a trial that is at issue, not merely one issue.
That ECHR does not approve a conviction after acquittal in which the convicting court did not hear the witnesses while the acquittal court did is true, but may not be as relevant here (the AK-RS case) as in other cases. It may have almost no importance with respect to Curatolo.
I am not disputing that Massei convicted; but if the ECHR were reviewing Massei alone, would the motivation report be considered reasonable or arbitrary? My reading of that motivation report is that is was full of arbitrary choices, essentially always in the direction of conviction, even when the evidence showed reasonable doubt.
If one accepts that Massei was not valid in terms of its violations of Convention Article 6, then Hellmann becomes the first valid trial generally in conformance with Article 6. And the Hellmann court engaged the independent experts Conti and Vecchiotti to review the DNA evidence; that is unique to that court over Massei. So Hellmann was not simply an appeal court in the US sense. Conti and Vecchiotti demolished the DNA evidence of the bra clasp and knife blade, as profoundly unreliable. Hellmann also demolished the other major points of Massei, although not very clearly dealing with the entry of a burglar by climbing up the grill over the ground-floor window or the "judicial truth" of multiple attackers.
The CSC in annulling Hellmann, on arbitrary grounds to my reading, whether legal or evidential, did not show any valid reason why the C&V report was not to be still considered extant. Did Nencini present a valid argument against the C&V report? IMO, not at all. Nencini then reverts to use of the bra clasp and knife blade alleged DNA evidence for conviction. He has "reasons" for the extra male DNA on the bra clasp, for example, but he has manufactured them with no evidence, only speculation, attributing the DNA to Meredith's boy friend and her girlfriends (although the non-Meredith DNA is apparently from several males, possibly including Raffaele). That is an example of arbitrary reasoning.
So here is my correction: the ECHR does not approve of the use of arbitrary reasoning to achieve a conviction after an acquittal, based on essentially the same evidence. It further objects to conviction after acquittal when the convicting court does not hear significant witnesses, or does not examine significant evidence, or arbitrarily deals with significant evidence, that the acquitting court has dealt with fairly, based upon the ECHR's opinion [the opinion of the majority of the Chamber or Grand Chamber panel of judges]. The ECHR judges against the Convention and its case-law, not against national law; it can find that national law, including constitutional provisions, was contrary to or was applied contrary to the Convention or case-law.