"Titanic" I'll skip because I don't think it was particularly that great. As far as "Best Friend's Wedding" goes I would definitely say that Julia Roberts is both the Protagonist and Main Character. I would say her efforts to subvert such wedding end in failure, but she is in a much better place. I would categorize that film as more of a Personal Triumph - the efforts to pursue the story goal (the problem everyone is dealing with) end in failure, but the Main Character is in a much better place.
Personally I don't think the goal everyone is dealing with in Casablanca revolves around whether or not he gets Ilsa, so I wouldn't categorize him as the Protagonist. It was mentioned previously that this was what Rick was pursuing and that this was the main goal of the story, so I was trying to use that as an example.
I have a whole series of articles I wrote on Endings last year here: Meaningful Endings
. Each one has a several video clips that help demonstrate my arguments. They should help explain that I don't think every story needs to have a happy ending (in fact, my favorites don't).
This goes back to what I believe is the incorrect assumption that the Protagonist is the character we care the most about, or empathize with the most. I believe that responsibility lies with the Main Character. When it all comes down to it this is an argument over definitions, and my argument is that the presently accepted definition of the Protagonist as being the character an audience empathizes most with is inaccurate.
Re: Ronaldinho, I agree that the relying on the ending to define your Main Character is absurd. I was using the ending of a story to help define who the Protagonist is. If you accept that idea that stories are about solving problems, then it follows that you would want to know whether or not those problems are solved and who was leading the charge towards him.