I like to consider the arguments of those who disagree with me. One point the pro guilt of Sollecito group make is that skepticism about DNA tests should apply equally to Guede as to Sollecito. This is a fair comment.
Let us assume that items contaminated with Guede's DNA were collected in an equally casual fashion to that contaminated with Sollecito's DNA. One similarity is that neither Guede's nor Sollecito's DNA was associated with identifiable tissue. A difference is that there are multiple samples that were positive for Guede's DNA, another is that the positive samples were obtained by different people at different times, there was DNA from Knox's clothes, and purse found by the scientific police and a separate sample from the post mortem obtained by the forensic pathologist from within MK.* But finally there is a question as to how Guede's DNA could contaminate the crime scene, he had never (legitimately) been in the flat before the murder, the police had had no contact with Guede, so how would contamination occur? The contrast is with one low copy DNA isolate of Sollecito in a mixed sample with other people's DNA. In a sample that was collected late after people had been in and out of the murder room, and the bra hook had been moved around before collecting. Gill suggested that the plastic gloves (he described these as being sticky for DNA), could have been contaminated from the outside bedroom door handle then transferred DNA when the bra hook was picked up in a gloved hand, handed around put down and picked up again. Other options are that Knox and Kircher may have shared a wash or a dry of their smalls with solicitors DNA transferring from Knox's to Kircher's clothes. Knox may have touched or sorted laundry of Kircher, contaminating them with Sollecito's DNA. Solicit may have touched them directly if they were hanging up to dry, Kircher may have hugged or shook hands with Sollecito then directly transferred DNA to her bra hook from her own hands.
* The palm print of Guede is sufficient in itself even if DNA typing was ignored.
So I think one could argue that there are routes for Sollecito's DNA contamination of the bra hook. What I do not think is reasonable to require 'proof'. In general on can disprove something, proving is harder. If the Italian courts are going to require 'proof' they need to be clear what is the definition of 'proof'. Is it on the balance of probability? Is it sufficient to establish reasonable doubt? I think that to argue Guede's DNA arrived by contamination on multiple samples when collected at separate times and places is very unlikely. I think there is a quantitative difference of many orders of magnitude in arguing Sollecito's DNA arrived in a single mixed sample of LCN level on the bra hook by contamination. Without some definition by the judiciary of what level of proof is required for contamination, it becomes impossible for any defence to meet the standard required. The judiciary ask an impossible standard of the defence.