... your basic idea is that it's all a dream. Firstly, I don't think a film-maker would do something like this unless it's for a very deep and meaningful reason. Considering Nolan has always claimed this is a heist film, making it all a trick destroys the jeopardy of most of the action and leaves the film intentionally hollow. Again, I'd be surprisied if that's what Nolan would want to do.
Also, remember that dreams are far, far more abstract than what we see in this film. Again, if this is all part of Cobb's subconscious, it seems like a failure on the part of Nolan's imagination.
So personally I think the film ONLY works if it is a real event and all these people are real people delving into the minds of others. The final image is just there to keep you guesing, and have you leaving the cinema with the same doubt Cobb and hiw wife had throughout the movie. It's Nolan final wink to keep us arguing like this!