There's also the whole "international politics" side. Remember a treaty is only good as long as both sides play along... it's not set in stone.
I'm sorry that I can't find this article, but I remember reading some time ago that there was an unrelated case in which a woman (a US-Italian dual citizen) had been convicted of a crime in Italy, got out on probation, and came to the US. I don't remember the crime, but the evidence against her was weak at best, and the State department agreed to extradite her on the condition that she be given only probation, not sent to jail. Once she got to Italy, the government uncrossed their fingers and sent her to jail. So (from what I read), they said there was a chance the US government would refuse to extradite Knox because the Italian government had not kept their earlier promises.
JCD is right- you are probably best off to contact a professor/lawyer specializing in criminal or international law.