Scenarios 2-4 cannot negatively affect Boolberry or its investors, so they have no investor risk. It is implausible that there is any way for CZ to take money from more honest people using Boolberry, so scenario 5 is extremely unlikely. Oligarchy is just as centralized as monarchy.
Firstly, oligarchy is not as centralized as monarchy. That's just wrong. An oligarchy of all would be maximally decentralized.
I think scenarios 2-4 are an investor risk because it removes upside from the coin. Adoption will be impeded. In the extreme case, adoption goes backwards until the coin coins to zero.
If one person is in control of what code runs the network, they can certainly extract value from the network by takings.