It is theoretically possible, but it is not realistic, if you consider the principles of ECHR. The ECHR acknowledgest that the right to counsel can suffer delays or limitation for cautionary reasons that depend on the need to pretect the effectivness of investigation, since the effectiveness of investigation is a public interest which has a need to be protected as well as human rights, and human rights are supposed to pay a price (be "partly sacrified") when they come into conflict with other rights.
It is not easy to establish a point of balance between conflicting rights. For example, it won't be easy to tell how long a delay to access counsel would be "reasonable" (48 hours is not 5 days - and Italian law allows up to 5 days). So not all "delays" are the same. But it is certalily easy to tell that the answer won't be completely slanted on one side, it's not going to be the absolute extension of the rights of an individual in detriment to the collective interest. If that was the case, even cautionary custody wouldn't exist; cautionary custody is a law by which a legally innocent person is kept in jail for cautionary reasons. The liberties of individuals are restricted for reasons of public interest.
It's not because it facilitated the prosecution, it's because there was a cautionary need to safeguard the investigation from a risk of being polluted.
Your reasoning about the five days is baseless: Mignini delayed the right to counsel until the investigating judge hearing, so that the defendants would have a chance to be examined by the investigating judge before their testimonies may get "polluted" by each other.
We are talking about three defendants who were arrested at the same time while they were in the process of accusing each other about an extremely serious crime, with Knox even attempting to communicate threatning messages ("blood on hands"): this was obviously the specific and exceptional circumstance.
I have not heard this before, can anyone expand on these threatening messages made by Knox, to whom? When? What was said and in what language?
To be strictly correct whilst you say
"We are talking about three defendants who were arrested at the same time while they were in the process of accusing each other about an extremely serious crime,"
but
1) Knox never accused Sollecito of any crime.
2) Sollecito never accused Knox of any crime.
3) Sollecito never accused Lumumba of any crime.
4) Lumuba never accused Sollecito of any crime.
5) Lumumba never accused Knox of any crime.
So you exaggerate in saying they were accusing each other. The sole 'accusatory' statement was Knox saying that Lumumba may have killed MK, but she was doubtful about the accuracy of this.
Were they genuinely arrested at the same time? Were Knox and Sollecito detained without being arrested until Lumumba was arrested?
Since only one made an accusation about one other, they could not all three have been arrested whilst in the process of mutual accusation.
When it comes before the ECHR the court will expect the defendant (Italy) to stick to the truth, changing the story, any conflict or uncertainty even so long after the event could be interrupted as guilt. Certainly exaggerations such as you have just made to justify detention without access to counsel, so that are brought before a court without having had a chance for their counsel to explain what is happening, advise or prepare a defence will be looked on poorly.
You have literally just said the reason for detention without access to counsel, is so that the defendants would go to court with no prepared counsel or defence, resulting in their prolonged detention prior to trial. That in Italy compelling reason for refusal to permit counsel is to deprive the defendant of the opportunity to prepare a defence prior to court appearance.
I do not understand how the possible discussion between defence lawyers (particularly as at that point Sollecito was viewed as not having been a participant in the murder), something that is neither unethical nor illegal, interferes with the process of the police investigation. Interfering with the police investigation is a positive action such as destroying evidence or bribing witnesses, it is not a lawyer advising their client of their right to silence. It is not preventing the lawyer taking instructions from their client.