As for the future part of the film being a dream I find this unlikely... I think the whole point of the movie would be pointless if it was just a dream.
Leone maintained that there were two possible readings. I don't support the dream theory and don't see why it's thought necessary to invent such a theory. None of the events which occur after 1933 are completely impossible and they could have happened in real life.
I think Leone was trying to introduce an element of ambiguity in the film. I've watched quite a few gangster movies recently and a recurring theme is that things happen and no-one knows precisely what happened in reality. The people involved are not always truthful, numerous rumors abound and people go missing and the truth never really comes out.
