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ORIGINAL_EPUB is not a backup, it is meant only for giving you the chance to revert an automated conversion gone wrong.
No one other than you should be messing around with your files. calibre does not have any options for artificially locking books -- it assumes that anyone who can run calibre and possesses file ownership of your calibre library has the authorization to do whatever they want.
As theducks pointed out, the Content Server allows others to access the library strictly for downloading, which should fulfill your needs.
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