I suspect the only way they were "contractually" obligated to do the ending from the book was by the fans of the books. If they had really changed the ending of the movie from the books the fans probably would have been furious. Many were upset enough as it is.
And, sorry, I don't see how this "twist ending" was a cop-out.
The movies have been extremely faithful to the novels almost to a fault, plenty of the lame and cheesy lines in the movies are lifted directly from the books. However, quite a few action scenes that were added to the movies were never shown in the books or actually never happened. This makes plenty of sense since movies are suppose to have more action.
I would have only considered it a copout if the battle had been completely meaningless and only there for eye candy, but it wasn't. In the movie, was one of the main reasons Aro decided to do what he did at the end. It enhanced the movie for quite a few people I think.
Also the writers did a great job having everyone stay in character during the 'battle' in the movie. Lots of great moments from the good vampires, the wolves, and the Volturi. I heard many cheers, laughter, and shocked reactions from the people I was watching with. I'm sure plenty of Twihards almost died of a heart attack while watching it!