Her boyfriend was obsessed with knives.
The term obsessed is charged and highly subjective. What makes something an obsession as opposed to an interest or aestethic, and how exactly did Sollecito fulfill these criteria? Besides, even having an obsession with knives does not equal having an obsession with using them on live subjects. I should know, i collect axes. I have committed exactly zero axe murders, nor do i have any inclination to.
Both of them turned off their mobile phones the night of the murder which was completely out of character for them.
When using inference from circumstantial evidence, you should always consider if there are plausible, innocent explanations. You cannot just jump to the most damning possible conclusion. Amanda said she turned off her phone to avoid being called into work and Raffaelle said that he didn[´t turn off his phone, but that his apartment had poor connection in several places. This turned out to be true. Their own explanations notwithstanding, this is incredibly weak circumstantial evidence. In part, this is because one could imagine many innocent explanations. Furthermore, if it is true that they turned their phones off to go an commit a murder, this suggests an organized crime whereas other circumstantial evidence used against them suggests a disorganized crime.
Amanda bought bleach the next morning.
This was claimed a year after the crime had taken place, by the storeowner. The people that worked there that day claimed otherwise, in line with what they had said from the beginning. They never found any receipt or the bleach bottle in question. Besides, this is irrelevant as there was no indication of any clean up or bleach usage at the crime scene or adjacent rooms (in any way that connects to the crime at least).
Her blood was found on the tap in the bathroom (guess she could have sprayed around her menstrual blood)
It was her own bathroom, that she used everyday. Her bodily fluids are expected to be found there. See again; jumping to the most damning conclusion/weak inferences.
She described hearing Meredith scream.
Unless you have a recording of the scream, which shows some unique aspect of the scream which Amanda described in a way that meant she had to have been there to hear it do describe it, it does nothing to show she wasn't pressured into giving that account. Mind you, people have been pressured by highly coercive interrogation tactics to admit to murders of people that were, in fact, alive. This is the gold standard of proving a false confession, and more generally that false confessions do occur in this way. Naturally, in most murder cases there are actual victims and so, whether a confession is genuine or not cannot be proven this way.
She said she saw her boss (a black man) and on it goes.
And Amanda says that this entire narrative was coerced from, and suggested to, her. These things happen and conventiently (for the police) the interrogation was not recorded. On principle alone it is my opinion that interrogations that are not recorded should be inadmissible as evidence, but that is beside the point.
More to the point is the fact that the police afterwards said something to the effect of "we interrogated her until she gave an account that we already knew was correct".