If you change your story, the probability of your guilt is 1.00. If one version of your story includes a false accusation (i.e., of another person), that goes up to 2.00

...and if your accusers are people who say things like the above, your probability of innocence is 10.00.

In all seriousness, you appear to be completely ignorant of the subject of coerced statements, as well as wildly overconfident about your ability to predict your own (not to mention other people's) behavior in highly abnormal situations. This, indeed, will make it difficult for you to understand the case for Knox and Sollecito's innocence.

If you change your story, the probability of your guilt is 1.00.

This is equivalent to stating "no innocent person ever changes their story." Empirically, this is false.

Let me provide a personal example. Several years ago, I was nearby when a co-worker was injured on the job. Two years later, I received a summons - she had ended up suing the company, and as an eyewitness, my testimony was relevant. Lawyers for the two parties asked me questions for a while; I answered to the best of my ability, as honestly as I could. My memory of the event was somewhat fuzzy by then, so I tried to only state things which I was sure I remembered accurately, and expressed my uncertainty when I was uncertain.

Once they finished their questions, they handed me a photocopy of a document in my own handwriting, written on the day of the event. The company had gotten such statements from everyone present on the day of the incident, you see. Not only had I forgotten that I had written such a thing until they showed it to me, my own handwriting directly contradicted the things I'd stated from memory minutes before!

This was before I participated in Less Wrong or was otherwise aware of the breadth of human cognitive failings. I was quite taken aback! My brain had betrayed me - not in old age, in my early twenties! I do not generally consider myself forgetful - if anything, I have an unusually good memory - yet in a fairly short span of time, my brain had managed to confabulate almost every detail of the event.

Since I became aware of it, I've noticed myself mid-confabulation on a regular basis. Despite how much my conscious mind values truth, my underlying hardware doesn't seem to much care.

You may indeed be one of the lucky people with flawless recall. This is not, by a long shot, a universal human trait. Memory is fragile. It is quite easy for many humans to misremember things without knowing anything has gone wrong, especially when under significant amounts of stress.

I know very little of the Amanda Knox case specifically, so this should not be taken as an argument for either side particularly. I am only arguing that your simple rule is not a good rule - it doesn't actually work.

Such a thing is extremely anti-social and is, I hope, criminalized itself.

I doubt that criminalisation would change much. Without having researched this, I would assume that people make false confessions when they believe they would be convicted even if they didn't confess.

I'm also not sure what's supposed to be so extremely antisocial about it. It's not like the police will surely catch the responsible party if only you don't make that false confession to get a more lenient sentence.

If you change your story, the probability of your guilt is 1.00. If one version of your story includes a false accusation (i.e., of another person), that goes up to 2.00

Well, one shouldn't be using such heuristics without prior research into the matter, precisely because of the typical mind fallacy. You couldn't imagine innocent people changing their stories - but it happens. So what does that say about the validity of your inference from your own imagination to the expected behaviour of other people?

If you change your story, the probability of your guilt is 1.00. If one version of your story includes a false accusation (i.e., of another person), that goes up to 2.00

At most you could convict them of something like obstruction of justice or criminal defamation, if you could actually prove mens rea beyond reasonable doubt.
But convicting somebody of murder just because they made an incorrect or even deliberately false claim, would not serve the interests of justice.

In this case, Knox was actually convicted of "calunnia" against Lumumba.
It could be argued whether this conviction was fair, since Knox made the claim in a statement signed at the end of a lengthy interrogation without an attorney, in a language she was not familiar with, and the statement mentions that she "remembered confusedly".
Anyway, fair or not, she already served her term for that false accusation.