Can files and objects be unlocked and unhidden after being locked and hidden?

Only by doing so in the bundle and then pushing an update

If they could they wouldn't be locked and hidden. If you have IP in an account you don't want to run the risk of having it exposed and this is NetSuite's way of ensuring that doesn't happen. If you want to see or modify it then you have to updated it via SDF or bundles to have that unchecked like Tristan mentioned