I'd given this 3-into-1 idea some thought myself so I'm very happy somebody is gonna do it.

IMO I'd open cut to Vader arriving on DSII early and intoduce Jerjerrod as the Moff and not Tarkin. Cushing is awesome obviously but his superiority to Vader would be hard to understand in an 120min movie. Plus if you're gonna cut him to the bone why not cut him completely.

Here's an idea to get round the Tarkin destroying Alderaan problem... open with Alderaan being destroyed by Vader. He could order it from on board his Star Destroyer and then we see him chasing Leia on the Tantive IV. I've always wondered what happened before the first shot of ANH so it would be cool to find out.

Perhaps shots from the Endor battle and a few select shots from the Coruscant ROTS battle could be used to show the battle above Alderaan. There are a number of shots of a rebel blockade runner in the ROTJ battle which would prove handy.

True I haven't quite figured out how you then resolve the problem of the boys heading off to Alderaan anyway but I'll give it more thought.