In view of the fact Italy has a corrupt and dysfunctional justice system, is it credible that a complaint about police misconduct would have gone anywhere. Would making a complaint against corrupt and vindictive police/prosecutors be risky as they may retaliate against anyone who made a complaint. The police/prosecution were allowed to get away with numerous acts of corruption and abuse such as an illegal interrogation which was not taped and access to lawyers were denied, feeding false information to the media, lying to Amanda she had HIV, committing perjury, destroying evidence, suppressing evidence and committing fraud. If the police/prosecution could get away with these acts, they would probably get away with complaints over mistreatment.
As your post states, a formal complaint of police mistreatment, directed to the corrupt Italian police or prosecutor who fabricated the Knox- Sollecito case, would likely have further endangered Knox.
The ECHR, in its case law, has recognized that persons who have been mistreated by corrupt police or prison authorities, who are under the control of those authorities, must often delay official complaints until the time that they have a hearing before a judge or the actual trial.
However, it also should be understood that this whole issue of an alleged lack of an "official complaint" is a diversion from the reality of Knox's case and the reality of Italian law and ECHR case law, whether the issue is put forward by Italy or a guilter.
Under Italian law, there are FOUR types of "official complaint":
1.
Denuncia: an oral or written statement by a private person
reporting a crime to a police officer, prosecutor, or judge, in accordance with CPP Article 333. When the police officer or other authority receives the report, that authority fills out a report and forwards that report to a prosecutor or other legally responsible authority, in accordance with CPP Articles 331 and 337.
2.
Querela: a written
request for the prosecution of an act considered to be an offense under the law, identifying the specific law, in accordance with CPP Article 336. A police officer or prosecutor forwards the request to a prosecutor or other legally responsible authority, in accordance with CPP Articles 331 and 337.
3.
Richiesta:
a request to a prosecutor for the prosecution of an offense from an official with authority, in accordance with CPP Article 342. This request may be in follow-up to a denuncia or a querela or to a police report (CPP Article 331) to the prosecutor.
4.
Istanza:
a petition to a prosecutor from a victim requesting criminal proceedings - that is, specifically naming the crime(s) by CP article number(s) - submitted by the victim or the victim's lawyer using the official forms, in accordance with CPP Article 341.
Knox's first official complaint was in her Memoriale 1, delivered to the police several hours after the interrogation of 6 November 2007 ended. That complaint was in the form of a denuncia. There were repeated official complaints by Knox and her lawyers after that, for a total of SEVEN, by my count of the ECHR's statements in the judgment Knox v. Italy. None of these official complaints were acted upon by the Italian authorities, although they had a legal obligation to act under Italian law and Constitution.
Furthermore, the ECHR does not require that a person file an "official complaint" in order to obtain an investigation by the state of a credible claim of mistreatment under Convention Article 3. Lack of a state-defined "official complaint" does not mean that the state is free of its obligation under international law (the Convention and ECHR case law) to carry out an effective and independent investigation of a credible allegation of mistreatment by the authorities. Under ECHR case law, the state must carry out that effective and independent investigation when it becomes aware of the allegation of mistreatment. See: Guide on Article 3 of the European Convention on Human Rights, Prohibition of Torture, Paragraph 120, p. 27 - 28.
Sources:
https://www.brocardi.it/codice-penale/libro-secondo/titolo-iii/capo-i/art368.html
The Italian Code of Criminal Procedure: Critical essays and English translation, ed. M. Gialuz, L. Luparia, and F. Scarpa, Wolters Kluwer Italia (C) 2014
https://www.echr.coe.int/Documents/Guide_Art_3_ENG.pdf
The section on the state obligation to investigate credible allegations of mistreatment begins with paragraph 113 on page 26.