So how come all the other housemates and Mez' British friends had similar long interviews and yet never once had the same visions of 'African immigrants' raping and killing Mez.
Because the police hadn't become fixated on any of the other housemates and friends. The police hadn't become fixated on any of the other housemates and friends because they all had robust alibis, particularly the two other housemates. Knox, on the other hand, had no alibi other than her boyfriend. And Knox had been acting in ways that the police felt were inappropriate. So once the police/PM convinced themselves - wholly improperly - that the break-in was staged, they inferred (wrongly, again) that only a keyholder would have a motive to stage a break-in. And, in the way that 2+2=5 when one is riddled with confirmation bias and tunnel vision, the police/PM quickly convinced themselves that Knox was the one who must have been involved.
And when they directed their focus onto Knox, they started to misinterpret everything from within this fog of confirmation bias and tunnel vision. They placed visual surveillance on Knox, and saw her meeting with Lumumba outside the university on 5th and having a conversation with him. This they feverishly misinterpreted as Knox and Lumumba holding a conspiratorial "catch-up" meeting to check on whether they were in danger of being discovered by the police. And when the police learned that Knox's mother was arriving in Perugia on 6th November, they determined that they had to "get" Knox before her mother put her on a plane back to the US. Hence the carefully choreographed dual interrogation on the night of 5th/6th November. "Witnesses" simply do not get called in to the police HQ for questioning at 10pm.
Once the police had pressured Sollecito to make a confused conflation of dates, they got Knox in. They soon discovered the now-infamous "see you later" text message from Knox to Lumumba. They thought this was the holy grail. They (thought they) had evidence of Knox arranging to meet up with Lumumba on the night of the murder - quite contrary to her repeated and unequivocal assertions that she was alone with Sollecito all that evening/night.
The police/PM were now utterly convinced that they had solved the crime. Knox had met up with Lumumba, had taken him to the cottage, and had let him in, whereupon Lumumba had assaulted and killed Kercher. Knox had cleaned up the crime scene and staged the break-in in order to deflect attention from her, and had lied to protect both her and Lumumba. And Sollecito had lied to protect the girl with whom he'd developed a whirlwind infatuation. Simples!!
The penultimate task for the police that night (penultimate to arresting Lumumba) was to get Knox to "buckle" and admit that the police's version of events was the truth. So that's exactly what they did. They used an unlawful mixture of coercion, threats, lies and "Sophie's Choice" tactics to break Knox into making her confused "confession/accusation".
That's how and why.
As for the police calling Patrick 'a dirty black': who informed the police it was him in the first place? An American student claiming she was scared of him.
Nope. The police knew that the recipient of Knox's "see you later" text message was the killer. That contact was stored in Knox's phone as "Patrick". Knox might technically have been the first person in that room to say the word "Lumumba", but only in response to the police asking who "Patrick" was. And it appears that the police told Knox - as part of their unlawful coercion strategy - that one of the reasons why it was so important for her to "remember" was that Lumumba was a very dangerous man who might well seek to silence the only living witness to his crimes, so if Knox could help the police to arrest Lumumba and place him in custody, she would be far safer herself.