Are there any feasible methods of maliciously reversing transactions? How can transactions be maliciously reversed? What resources are required?
This is not a possible thing to do at all without having over 51% of the network. All transaction are irreversable.

With 51% of the network hash power, it is really easy to reverse transactions.

Anyone with that kind of power would make a better profit just mining as attacking the network would cause a collapse in confidence and drop the value (with that kind of power its safe to assume you have a hoard of BTC)