Thanks for your quick response, and for also modifying the po file to add some explanations.
With these questions sorted out I am proud to announce that the Dutch translation of calibre is 100% complete, and has been fully checked for errors.
On a final note, Mark Dikkenberg (
https://translations.launchpad.net/+groups/calibre) is no longer active in translating calibre, so it might be better to appoint Launchpad Dutch Translators (
https://launchpad.net/~lp-l10n-nl) as manager of the translation of calibre.
Thanks again for your help, and for creating calibre in the first place!