In an ECHR case Italy will be the defendant. The problem is taking a reductio ad absurd approach, if defendants were never permitted counsel prosecution would be easiest. Assuming one does not take that view then any restriction to counsel has to be justified, as you said specific and exceptional. Because of the equality of arms issue, if the prosecution have access to a lawyer (the pm) the defendant should have access to a lawyer. The ECHR can certainly rule that particular provisions of Italian law are in conflict with the human rights. Although you say the restriction of access to counsel was legal under Italian law because it facilitated the prosecution, you can give no reason why it was necessary other than preventing access to counsel disadvantaged the defendant. This case could be made in any case. If there was really true cause then mignini would have gone to court to justify the continued restriction to the five days allowed. that he failed to make the case shows there were no good grounds. The ECHR is clearly not going to favour the routine deprivation of access to counsel because it makes prosecution easier. The circumstances should be specific and exceptional. The grounds should be documented and challengeable. That the Italian courts failed to accept the argument is exactly what justifies the case going to ECHR.
It is important that the "good reasons" for deprivation of counsel during an interrogation are in reality "compelling reasons" in the ECHR language.
From Salduz v. Turkey:
55. Against this background, the Court finds that in order for the right to a fair trial to remain sufficiently “practical and effective” (see paragraph 51 above), Article 6 § 1 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 (see, mutatis mutandis, Magee, cited above, § 44). 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.