Well, I see three issues here.

1. The rate. If you were quoted a rate then that is what they should have charged you. If they charged you other than what was quoted then you have an issue - otherwise it is open and shut. I don't see how adjusting it to some "average" rate would ever fly.

2. The power failure. I would consider this a serious inconvience. Even though it may not be the hotel's fault, you should expect a certain level of comfort. Some compensation may be in order: extra MR points, a voucher or even a comped room.

3. Accusing you of going behind the desk. Obviously something serious happend that upset the staff, even though it was not you. They don't need to badger you, however.

A well worded letter to Marriott corporate (and copying the GM), as camachinist suggested, for items 2 & 3 would be in order. You should quote names for item 3.