The prequels seem like such a...
...to clear. Let's hope he doesn't get tripped up.
That's easy. Movie starts with various shots from the prequel movies, from little Annie, to Django Fett, to the murder of the children, to the "Nooooooo!" etc. Then we see old man Han wake up in a sweat. Old woman Leia wakes up and comforts him, "What was it?"
Han: I just had the most horrible dream. There were stupid looking monsters, Darth Vader was this annoying kid who later killed a bunch of children. Terrible.
Leia: Go back to sleep, Han. It was all a dream. A terrible, terrible dream.
*Han smiles, kisses Leia on the head, then goes back to sleep; cue main credits*
I don't like flashbacks either, not in Star Wars. It breaks the fairytale story/saturday morning serial approach. Lucas had a very controlled and specific editorial approach to these films. All of them OT and PT. The screen wipes, the approaching planet shots when location changed, all based on old serials, flashbacks are not a simple storytelling device in this context.
Regarding the Disney aspect of these movies, they will be kiddified, like Rebels. That doesn't mean a bad movie, but there are just too many damn toys to sell to eliminate ANY part of an audience with darkness or violence. Overall I would expect Return of The Jedi levels of tone, with a less scary emperor. Kids need to want the toys.
Disney wouldn't have allowed any of this stuff, don't expect:
I can see an opening scene flashback, a la Star Trek, but wouldn't expect to see flashbacks throughout the film. But even there it's possible that you'll have someone telling a story and the screen will go to the flashback, like you saw with Lord of the Rings sometimes (I think). I don't think they would make it very complicated or difficult to follow if it happened. Going to Os Corp's point, I doubt JJ will stick too religiously to all the filmmaking decisions of the OT, so I can see him not trying to ape the "serial movie" aesthetic too closely.
As for the violence, I don't know. Return of the Sith had some pretty terrible imagery as you point out (in terms of being extreme, not just being terribly bad, though it certainly had that as well), yet I'm sure they had no problem selling toys out the wazoo. I don't know if the new films need to go that far as seeing on screen beheadings or anything, but a severed limb? Maybe. An off-screen beheading? Maybe. Charred bones? Maybe. Trying to sanitize things too much might be off-putting to the old bastard fans who go to see this, and though not the primary audience Disney is going for, that's definitely a group they want to appease. I imagine we'll see a compromise, and that there will be real threat, conflict, and pain in this film. At least, I hope so, because the authenticity of those dynamics and emotions in the original films are a huge part of why they were so good.