I have another case to add to your three.
I am the archnemesis of the Bitcoin network, and I am trying to do scenario iii above. As part of this plan, I want to - let's say - double the current difficulty. Theoretically, the network is currently solving blocks every 10 minutes on average. To double the difficulty, I need to double the rate at which blocks are being solved. To do this, the computing power of the network needs to be double -- or, in other words, my own computing power needs to be equal to the power of the rest of the network combined. Quite a feat, but possible for a well-funded archnemesis such as I.
However, if I had enough computing power to do this, I think I would use my evil strength to bring down the network in another way. Since I have half the computing power, I could start forking the real block chain and go on a double-spending spree. If network users couldn't depend on the network guarding against double-spending, the whole Bitcoin system would be doomed, I think (no one could safely receive BTC, so there wouldn't be much use for it).