You didn't answer the question; there is still an oracle problem. How do I know you're holding a unit of the index fund for each token? I have a pretty good idea my broker will, as he'll go to jail if he gets caught short. That's usually right, but then we have clowns like Jon Corzine who get away with it because they're more equal than others.

The other stuff can be done with a permissioned blockchain; if you can adequately solve the oracle problem, you'd have something pretty interesting. If you don't, you're an unregulated broker who uses Chaumian tokens.