Now, if it is the case that she didn't, then it follows that, given sufficient information about how-the-world-is, one's probability estimate could be made arbitrarily close to 0.

What, like 1/3^^^3? There isn't that much information in the universe, and come to think, I'm not sure I can conceive of any stream of evidence which would drive the probability that low in the Knox case, because there are complicated hypotheses much less complicated than that in which you're in a computer simulation expressly created for the purpose of deluding you about the Amanda Knox case.