Is it considered cheating to amend answers based on what colleagues have published, knowing that the nature of the exam allows for open books and a long completion time, and the university has not warned against cheating?

The university allowing the consultation of books during an exam does not imply the permissibility of copying answers from peers, because the questions require understanding and inference. Copying answers from peers is cheating, which negates the benefit of the exam and contradicts its purpose. You must repent for what you have done by intentionally reading and copying answers from peers during the exam, as it is forbidden cheating.