No. They could easily have the movie the same length as it was, yet have it
actually have something to do with the original story. Not that hard. Me and my friends spent the entire movie pointing out where they messed up, how badly they messed up, and how much easier it would be to fix it and make it look better. Also: Prophecy: Eragon is most likely
not Morzan's son, rather Brom's. Either the first Eragon's sword, or Brom's will be found under the Menoa tree. Murtugh and Thorn are possessed by the beings that inhabit the vault of souls, which is how they match Eragon and Saphira in power. Galbatorix manages to grow in strength due to the reason outlined below. End of speculations.