I've not had the sorts of problems you have, but the whole messing about with the Arkham City PC release seems too much for them to handle. I'm not sure if it really was going to be delivered '6 weeks from 18th November' but when I weighed up the factors, I'd rather pay a little more to guarantee I get it on release (or possibly a day early), have better DLC and don't have a SteelBook.
Cancelling mine was easy enough since I've had no problems with seeing my order history. Just clicked to go back far enough in the history, clicked the specific order and clicked to cancel. Then had to wait a short while to receive a cancellation email. The website is not the great though.