Since putting a block in an accounts's chain requires it to be signed by the account owner, how does the stake key holder sign blocks for an account it doesn't have the key for?
It doesn't. But voting with historical stake will no doubt present the relevant problem.
This attack is feasible for systems that grafted voting on top of a bitcoin style block chain where transactions are unordered. With RaiBlocks an account signs a total order for all transactions it generates.
Rewriting history of an account can only be done by the account owner themself, representatives can't do this.