Kevin, the problem is that we don't agree on the "facts". All the narratives we have given are shot down by your assertion that Meredith died hours earlier, or that the dna on the bra is faulty, etc. So I think it's unfair to say that we haven't offered a narrative, we have. You just don't agree with it.
If you can't provide a narrative which is consistent with the scientific facts, then my job is done. I've demonstrated that your belief in the guilt of Knox and Sollecito is faith-based and irrational.
If you want to keep holding that faith-based belief, I can't stop you.
What I would like to see, and of course this is only a request, not a demand, but I would like to see a narrative from you guys that explains why a police force and a justice system would falsify evidence and dna just to send two innocent young adults to jail. Not just any innocent young adults, but one who's father is wealthy and somewhat powerful, and another who is American and bound to be supported by many from her home country. They had the supposed murderer within weeks, the public would have been satisfied, its not like there was pressure to "catch the killer who is on the loose", they had the killer soon afterward. Anything is possible, but it's going to take a lot for me to believe that.
You're asking the wrong question, I believe. We've cited multiple cases (the Norfolk Four, Stefan Kizsko) of police doing exactly that. So the fact that you find it strange that police sometimes do so is irrelevant. Police do it sometimes, and our job is to figure out whether or not they did it in this instance.
However to answer the question you did ask: Speaking broadly, a serious historical problem in policing in the UK and Australia seems to have been that when you get a police force who aren't all that bright, arresting criminals who 90% or more of the time are guilty of what they have been arrested for, they can develop a culture of faking evidence ("verballing" was rife in the UK and Queensland in some periods), beating suspects, fabricating evidence and so forth very easily, and it's often hard to tell the difference in outcomes between a corrupt force working this way and a clean force doing their job properly. Either way the crims go down, so it's only when the police get caught red-handed setting up an innocent person that they get sprung.
I suspect that something similar happened to police culture in Perugia - lacking any checks on them, a culture developed of assaulting and browbeating suspects, twisting evidence to suit the prosecution, producing verbal statements of convenience, possibly falsifying DNA evidence, leaking damaging slime to the media and so on. Similar things happened in the seventies and eighties in Australia in some parts of the police force.
So in a culture of police bending the rules, there's a worst-case scenario of a prosecutor going off the deep and the police enabling him in his fantasy. Mignini decides early on that it was a three-way killing because the man is just not stable (as was documented in the Monster of Florence case, despite Machiavelli's cherry-picking of irrelevant factoids to try to whitewash the nutter's reputation), the police buy into his story and start finding "facts" that support it, they browbeat an internalised false statement out of a vulnerable young woman, they haul in Lumumba, and for a minute they really think they've cracked the case of the century.
Then the forensic evidence starts coming in and it's an "Oh ****" moment for them. No sign of Amanda or Raffaele in the murder room, no positive evidence of a staged break-in, no positive evidence of a clean-up, an alibi backed up by unimpeachable computer evidence that places Amanda and Raffaele at Raffaele's house on the night of the murder, over a dozen witnesses supporting Lumumba's alibi including an international professor who travels back to Perugia to get him out of prison... it's not looking good at all. Every bit of their case is garbage and they have nothing to go on to find the real killer.
This is the point at which honest cops might have blown the whistle, outed Mignini as deranged and tried to salvage the situation. That didn't happen. So things started to happen to salvage the situation the other way, to save face by getting a conviction regardless.
The hard drives get fried - there goes the proof of their alibi. Rudy is arrested and like a murderous Lego block he is slotted into exactly the spot in their story that they had originally marked out for Lumumba. Stefanoni makes a highly irregular trip in person to Perugia to rustle up some new evidence and hey, presto! Raffaele's DNA is found on a bra clasp... or at least
some peaks on a graph look like peaks on his graph, and that's good enough right? The knife is tested and retested until Amanda's DNA shows up, without any of the precautions and replication normally used to prevent false results from contamination, and the logs that could tell us whether Amanda's DNA was floating around that lab that day are concealed from the defence. A series of outright lies are leaked to the public about Harry Potter books and bleach receipts and so forth to poison the jury pool.
Remember Meredith died on the 2nd of November, and police arrested Amanda, Raffaele and Lumumba on the 6th. Rudy wasn't even on the police's radar until the 19th, and Lumumba wasn't released until the 20th. There was a solid fortnight when the police had been telling the public "case closed, we solved this, we have all three murderers, we're good" when in fact they had three innocent people that they were railroading and no idea who the real killer was.
Why didn't they just admit when they bagged Rudy that they had been completely wrong, that all three of their initial suspects were totally innocent, that they'd browbeaten a false statement out of Amanda, arrested Lumumba based on nothing but the false statement they elicited and generally made complete asses of themselves? Because people don't do that. In fact the community you've chosen for yourself over at PMF is an excellent case study of people's resistance to changing their minds in the face of new data, if the new data is not what they want to hear and changing their minds would mean an embarrassing about-face. People don't like to be embarrassed, and given the choice of embarrassment or railroading two totally innocent young people into murder convictions the Perugia justice system took the path of least embarrassment.
So the very short summary of a very long question is - you want to know why they stuck to their beliefs in the face of irrefutable contrary evidence? It's the same reason Michael/Fulcanelli will never stop preaching his faith. It's the reason you decided to believe that we were lying to you about the facts of this case, rather than accept that the facts about the case were not as you wanted them to be. Most people don't change their minds when they are proved wrong, they just get angry and defensive and stick to their guns even more tightly.
The people who don't do that - the people who proportion their beliefs to the available evidence and change their beliefs when new evidence is found - are called skeptics.