I've noticed a lot of things that cut down on runtime (in a good way, of course.) Just in case the movie is dramatically shorter, have you considered using scenes from the other two prequels? My wish would be to turn them all into one film with ROTS as the base since it's the best one, and I'm working on that on my own but my edit is definitely not going to have all the cool things this one will. Not necessarily flashbacks but certain scenes, especially with Palpatine, fit in the context of all of them.