Graham, I want to thank you for bringing this film up. I finally watched it and it has to be one of hardest films to decode that I've ever seen. I interpret it as everything having been a part of Diane's dream, although it could also be a recollection of how she might have wanted her life to be in the final moments of her life.
Watching it felt like being in a dream. It was breathtaking. I can see why it's your favorite film. It leaves you hanging.