It's worth reminding ourselves that the ECHR cannot constitutionally order acquittals or retrials. But actually, in practice it has the power to do just that. If the ECHR rules that a person's human rights have been violated in relation to a criminal conviction, it can order the state in question to apply a remedy. The state in question is legally bound to comply with that order, unless it elects to withdraw as a signatory to the convention.
In Knox's criminal slander case, therefore, if the ECHR rules that Knox's right to a fair trial was violated, it will order the Italian state to apply a proper remedy. In practice, this means that Italy will either have to quash the conviction (and almost certainly pay monetary compensation), or order a retrial on the charge, excluding any elements that related to abuse of Knox's human rights. And since the state's case against Knox would crumble to almost nothing if she were retried with an exclusion of those areas which related to an abuse of her human rights, she'd without doubt be acquitted in such a retrial anyhow. Which, in turn, means that pragmatically, Italy would be almost certainly bound to quash the conviction totally without going through the farce of a retrial and acquittal.
The only other choices Italy would have would be a) refuse to comply with the ECHR remedy order - which would spark a significant constitutional and judicial crisis. or b) withdraw as a signatory to the European Convention on Human Rights, which would also have significant wider ramifications for Italy and Europe.
In short, if the ECHR finds that Knox's human rights were violated, this would in practice mean that Italy would have little choice but to quash her conviction.