If an Attacker Controls 100% of the Hash Power, Can They Change the Block Reward?

No, even with 100% of the hash power, an attacker cannot unilaterally change the block reward. The block reward is a fundamental rule of the protocol, defined in the network's software.

All honest nodes on the network would reject any block that contains an invalid block reward, as it violates the established consensus rules. The attacker's block would be considered invalid and would not be propagated or accepted by the rest of the network, meaning their 100% hash power would be wasted.