Do you have evidence that "they actually believed he was involved"?
If this was a legitimate case of such belief by the police and prosecutor, why did Mignini alter Amanda's text message in the Nov. 6, 2007 arrest warrant, by deleting the exculpatory part not consistent with a meeting? That is, Mignini changed the text message: "Certo. Ci vediamo più tardi. Buona serata" to: "“Ci vediamo dopo” in the document providing the reasoning for the arrest. Leaving out the closing farewell (Buona serata, [have a] good evening) changes the meaning.
Source:
http://www.amandaknoxcase.com/the-interrogation
The point of Stefanoni's "carelessness" in handling the bra clasp with her dirty glove may have been to transfer DNA, but actually that is a hypothesis. It's actually unknown where and when the DNA of 3 males, one of which may be Sollecito, entered the sample allegedly obtained from the bra clasp.
Besides the questionable collection method and timing for the bra clasp, which was such as to make its DNA content, if any, inadmissible in a fair trial, there were concerns in the laboratory procedures that suggest contamination in the lab and possible data tampering. The two positive control samples associate with the run with the bra clasp sample were intended to each have 23 pg/uL of DNA. However, the results provided by Stefanoni showed that these positive control samples actually contained an average of 108 pg/uL of DNA -
5 times too much! Where did all this extra DNA come from? It's obviously from a contamination event in the lab itself. And Stefanoni did not provide valid negative (zero DNA) controls for this batch; the ones supplied did not have ID numbers that matched the run, and apparently were derived from a different test date. In addition, there are anomalies in the number of the the test samples for the bra clasp indicating that the results presented are from a rerun, with the data from the first test suppressed. Stefanoni did not discuss or explain any of these discrepancies, which according to "equality of arms" and "providing all facilities needed by the defense" - rules of the European Convention of Human Rights as interpreted by ECHR case law, she would have been required of the prosecution. As far as known to the public, these runs were not recorded by Professor Potenza. Finally, any DNA on the bra clasps was destroyed by Stefanoni after testing, as she stored the bra section with the clasps in a sealed test tube with an aqueous solution (probably extraction fluid) which would have degraded the DNA.
Sources:
http://www.amandaknoxcase.com/bra-clasp-contamination/
http://www.amandaknoxcase.com/meredith-kercher-bra-clasp/
And as additional considerations: 1. The bra was apparently torn off from Meredith Kercher's body, in a manner that did not involve touching the clasps, so any DNA on the clasps would not necessarily be associated with the murder/rape of Kercher. 2. Removal of a bra by undoing the clasps does not necessarily involve touching the clasps themselves; the usual procedure is for the two fabric parts on either side of the fasteners to be moved toward each other to unfasten the clasps, freeing the hooks from the eyes. Indeed, this is the usual method to undo hook-and-eye clasps, because the tension force holding the hook against the eye must be overcome, and the curved segment of the hook must be pulled clear of the ring of the eye, to open the clasp.