Getwork returns data (essentially block header) and midstate. Midstate can be created from data. Data contains block header which MUST contain previous block hash. Block header contains also merkle root which is unpredictable but everything else must be consistent.
Hash is unpredictable but the data to be hashed must be valid. Otherwise the block will not be accepted by the network.
Right. In theory it should be possible to modify miners to check this previous hash. For some reason I only remembered the merkle root was here.