A note on the legal processes and the status of Knox and Sollecito:
1) Knox's/Sollecito's current status
Knox and Sollecito are currently considered innocent in Italian law. They will not be considered guilty unless they are ultimately convicted by the Supreme Court ruling. They are currently in prison, but not because they are serving a prison sentence for the crimes. They are in prison because they have been remanded into custody pending the outcome of the trial process. And they've been remanded because of a) the gravity and nature of the charges against them; b) the risk of reoffending; and c) the assessment that they pose a potential flight risk. Knox and Sollecito are not therefore in the same category as convicted prisoners: this is almost certainly why they are accorded certain privileges that are likely not afforded to convicted prisoners, such as a high number of visits.
2) What happens upon acquittal?
When Knox and Sollecito are acquitted, they will almost certainly be released virtually immediately. It's possible that certain restrictions might be placed on their movements pending a potential prosecution appeal the the Supreme Court, but I think it's more likely that they will have no restrictions placed on their movements. I think Knox will therefore be free to return to the USA, on the understanding that she might have to return to Italy in the event of further legal proceedings.
3) If they are acquitted, what will happen in regard to the Supreme Court?
The prosecution has the right (but not the obligation) to appeal to the Supreme Court, but only on points of law and not on findings of fact. If the prosecution does decide to appeal, then the Supreme Court will hear the case and make a ruling accordingly. Kno and Sollecito will almost certainly not be required to be present for any Supreme Court issues.
4) What if there's a prosecution appeal to the Supreme Court, and it's successful?
If the prosecution were to appeal to the Supreme Court, and if the Supreme Court were to rule that Hellmann's court had indeed made errors in the application of law, this doesn't mean that Knox and Sollecito would have their acquittals reversed. In such a scenario, the Supreme Court would refer the case back to the appeal court level - i.e. Knox and Sollecito would have a retrial at the appeal level. Both Knox and Sollecito would be required to appear back at court for a new appeal trial, which would be held under exactly the same circumstances as the current Hellmann trial. I think it's unlikely that Knox or Sollecito would be taken back into custody at this point: I think they would remain at liberty until the end of any new appeal trial. If they were found guilty in this repeat trial, they would of course be taken back into custody.
Having pointed all this out, I might add that this is, in my view, purely hypothetical. I don't see any current grounds for the prosecution to appeal to the Supreme Court on issues of law. My view is that the strong likelihood is that Knox and Sollecito will be acquitted in Hellmann's court and released, and that will be the end of the matter for them.