Goodbye duvet cover. Even after swimming around in water and me trying to work with it in the water (I will keep the suggestion by @elbutterfield in mind for less extreme cases), it wouldn't untwist. I chopped it up. Inside were the following:
Two bath towels
Three pillowcases
Two hand towels
One dishtowel
Two dryer sheets
@publickovacs Oh no! I am so sorry to hear this. That certainly is an extreme case of twisting and bunching. I wish my solution could have worked for you this time. You strove valiantly and should get a medal for the attempt!