Hi Valkrist,
Sorry to hear about the issue! The part where you said that even Steam was unable to verify the files suggest me that it could be an issue where the file got stuck in the file-manager in Windows and it was unable to overwrite it. I guess restarting your PC should fix this problem, and then either the in-app downloader or Steam’s verify system should be able to fetch the file from the servers.
Please let me know if it still doesn’t solve the issue.