1) libunrar.so is not sufficient, that is the library version of unrar but you
actually need the python binary extension python2-unrardll ("/usr/lib/python2.7/site-packages/unrardll/unrar.so") which will in turn link to libunrar.so because shared libraries.
2) I guess Debian could put both packages (libunrar and python2-unrardll) in the nonfree section, but calibre would still need to depend on it (python2-unrardll), because if it
isn't installed, then you will still get that error message, which is unfriendly to users.
If Debian wishes to both handle the DFSG (no comment) and allow people to install from the nonfree section to get RAR comic handling in calibre, they will need to provide their own downstream patch to supply graceful error messages, and hopefully add an optional dependency on python2-unrardll. (Sadly, your comment "in any way" seems to imply that Debian would expect their users to simply learn through the grapevine that python2-unrardll has anything to do with calibre.)
calibre itself will of course not support that error message itself, since downstream packagers are not "supposed" to strip out parts of calibre functionality just because of their political differences.
