State control is not necessarily banning. For instance, the Chinese gov could make a deal with chinese miners, that, against electricity subsidy, they exclude certain addresses or so, and orphan blocks containing these transactions. If these miner pools have more than 51% hash power, that becomes an unspoken soft fork that will block these addresses. For the moment, the Chinese government should make an agreement with 5 persons: the 5 biggest pools.
I don't think so. Even with 51% hashing power you wont be able to mine all blocks. So some blocks will be mined by non chinese miners and they will built on this blocks. invalidating 2 or more blocks thanks to the chinese will be bad for bitcoin and its price. People will simply go with another coin and i doubt that the miners will be happy about that, so they will not agree to blocking certain addresses.
Well, if they have made you an offer you can hardly refuse, you will orphan them, because if you don't, they can verify this. With 75% of hash rate agreeing on the secret hard fork, the orphaning is not that big. Chances for 1 bloc orphaning (normal) is 1/4. Chances of 2 blocs orphaning: 1/16. Chances of 3 blocs orphaning, 1/64. Chances of 4 blocs orphaning, 1/256. So, essentially every 1000 blocs, you will have 5 blocs or more orphaned this way. Not a big deal, especially because most transactions will be taken over in the new chain prong. So as a user, you wouldn't even notice if you're not concerned. You had 5 confirmations on the chain ; boing, these are orphaned, by another prong where you ALSO had 5 confirmations. You wouldn't notice.