I think the diamond diadem is almost a coronet in its own right, but diadem plus coronet sounds like a good solution. It would clearly identify her position. I wonder what a princess consort coronet looks like. Was Prince Phillip's different from the coronets of the other royal Dukes? Would Camilla's be the coronet she is entitled to as a Duchess?
Edit: I agree that she would not proceed to a Coronation bareheaded unless she was to be crowned.