We tend to look at this case with the benefit of hindsight. And, so, since Amanda incriminated herself on the 5th and 6th interrogation night, we consider the effects of her treatment from when she arrived at the questura that night and whether her procedural rights attached at the moment her final interrogation began or somewhat later.
And yet it is clear her procedural rights attached days earlier. As you point out Amanda was a suspect "more or less at once". And not merely because the investigation was at an early stage and in some sense it could be said that "everyone is a suspect". That she was meaningfully a suspect is demonstrated by Ficcara and Mignini's statements on the subject.
She underwent approximately 40 hours of questioning prior to the 5th and was not simply "at risk of unfairness" but actually treated unfairly. She was required to attend the questura, subjected to lengthy interrogation, such interrogation being hostile in nature. On the 4th November, she was subjected to an eight hour session and as she relates in a bugged conversation with Annie Fuller on the 5th:
"It’s a little frustrating because... that is, they are very stressed and become more impatient with me and then... yesterday it’s been very difficult because when I went to the Police Station they questioned me and when I tried to reply they were telling me: " Are you’re lying? Are you sure you’re not lying? Because if you’re lying you are getting yourself in big trouble", and I: "I’m not lying, I’m trying to help you", you know? It bothered me, but at the same time when they brought me... There, they brought me home again and they asked me to examine all the knives to see if any were missing and just the fact of being there (in the house) frightened me to death, not to speak of having to browse through all the knives..."
Tragically, the unsullied, naiive and inexperienced Amanda lacks the radar that would have guided her straight to a lawyer's office. Yet, her rights are clearly enunciated in Salduz:
" n order for the right to a fair trial to remain sufficiently “practical and effective” ..., Article 6 § 1 [of the Convention] requires that, as a rule, access to a lawyer should be provided as from the first interrogation of a suspect by the police,unless it is demonstrated in the light of the particular circumstances of each case that there are compelling reasons to restrict this right. Even where compelling reasons may exceptionally justify denial of access to a lawyer, such restriction – whatever its justification – must not unduly prejudice the rights of the accused under Article 6 ... The rights of the defence will in principle be irretrievably prejudiced when incriminating statements made during police interrogation without access to a lawyer are used for a conviction.” (Salduz v. Turkey, Grand Chamber judgment of 27 November 2008, § 55)."
That Amanda did not incriminate herself before the 5th November deflects us from paying much attention to her treatment between the 2nd and 5th November when she was being softened up and broken down. Yet, not only would the early presence of a lawyer have protected Amanda for that period of time, it would have ensured that the interrogations of 5/6 November did not take place and arguably would have re-railed a derailed investigation.