A police interrogator slapped Amanda Knox on the head if he didn't like her answers to questions. A suspect can become totally confused and tends to try to satisfy the tormenter in order to make him stop. (see links I gave above). Al this without the presence of a lawyer.
I'm sure the jury (includes magistrates in Italy which has a modified Napoleonic Code) took this into account when they deliberated the evidence and threw out statements not collected properly.

DNA evidence would be difficult to evaluate in this case, since the two women lived together and Sollecito was a frequent visitor. DNA of all three would be all over the place.

It also seems forgotten that a third suspect is serving a sentence for the killing. He had no close connection to either Knox or Sollecito.