Unless I'm misunderstanding things (which is quite possible) then it seems to me that it's also a much much bigger issue than the time it takes to confirm a transaction. Doesn't control of the block chain enable the controller to reverse every transaction (or at least a significant proprotion of them), and also create fraudulent transcations?
How many transactions he can reverse depends on how much computing power he has. The more transactions he wishes to reverse, the more he needs. (And the software can be tweaked to make it as hard as we want to reverse larger numbers of transactions without any change to the protocol.)
Control of the block chain gives you no ability to create transactions. At the very worst, you can block transactions and undo transactions. Nothing stops the recipient from redoing an undone transaction so long as a conflicting transaction hasn't been placed by the original sender.