I think that the secret can still be preserved easily. It works pretty well within what's been given (you could make a fan-edit of the existing ROTS and keep it ambiguous). You can at least keep it unsure until it's revealed in ESB.

I think it would be more the 'prequels' and not the first three chapters of the story if you revealed everything. Meaning, often times we think of prequels as a part of the story told AFTER the events before it have already come out or been told. However, they're marketed as the first three chapters of the story, in which case way too much is revealed.