The only shot this film has of not being a complete laughing stock, is if Lucas allows for Steven or whoever to truly confer on the story outline. Lucas's ridiculous insistence on the MacGuffin and various other story elements for KOTCS kept the movie from being made for over a decade. Not to mention those elements were roundly criticized.
I think its a good thing, if Shia is on the ball. I say this because it would mean Lucas would have crafted a story, and gotten Spielberg and I assume Ford, et al., to agree at face value. All within less than one year. Now getting a script they agree on is another thing, but the reason KOTCS went through so many scripts was because Steven and Harrison were not comfortable with the alien story Lucas pushed on them. If this is a general story they are happy with, not worried over, then the script should be a piece of cake.
As for whether Spielberg, Lucas, and Harrison can do the film..... Lucas is really not as busy as he says, in fact he's probably the least of them all. Harrison only works for a portion of the year, and his only film for 2010 and beyond is Morning Glory, which I think they are just starting to film. Spielberg has the most on his plate, but he also literally has an army of people who work for him. He'll find the time.