I repeat: no. It is not true that the ECHR case law extends up to an the absolute category encompassing any possible situation of any person in custody without a lawyer and any possible statement. It seems you like to pick up cases to deduce the existence of categories that you stretch up to the most absolute set in order to cover under the same umbrella any possible case with just something in common.
But this theory is disproven even by very simpe examples such as Luciano Aviello: a person under judicial custody, who is not assisted by a lawyer, releases statements before a magistrate as a person informed about facts, and is later charged based on those statements. Are you saying the ECHR would declare those statements inadmissible as evidence against him for the charge of cakunnia? This is obviously unrealistic.
Even as for common sense, the substantial rights of Amanda Knox were clearly not violated. Even if the interrogation was considered not good evidence for a charge of calunnia, Knox was aware about her right to sikence before releasing the 05:45 statement, and her written memorials were voluntary. She was not a suspect not even informally, because the investigators could not be aware that a calunnia was being committed. Calunna is a crime under any circumstance no matter what others behaviour is, no matter if procedures are correct or not, just like any other crime, and the ECHR never disputed the nature of this crime. Actions committed during police interrogation or during custody are not protected preliminarily by any special licence or privilege, citizens always have to respect the law even under custodywithout lawyer, so the whole point is moot.