Once the encryption schemes are broken, it's not just posterity, but every malicious actor with access to encryption-breaking tech will have a field day.
I don't mind a large collection of data about me being made available to historians, I just mind that happening with my contemporaries.
Yes. Former presidents do not lose their benefits if they're convicted of crimes and sent to prison; changing that would take an act of Congress, and there's probably not enough political will or support to curtail the executive branch in such a way.
If he's actually given a prison sentence and not home confinement, though, I feel that they'd likely hold him in a military prison- for his own safety, to keep an eye on him to make sure he doesn't keep committing crimes or attempt to flee, and to dissuade jailbreak attempts by outside actors.