@Samurai
1. Adding books - If you stop using calibre and uninstall it, it will NOT delete the library folders, in fact if you Remove a library it will NOT delete the library folder, it simply removes from its list of known library folders. If you want to keep a record of the original file name the
GetFileName plug-in will do that.
3. I believe some people use the Search the Internet plugin to get 'better covers'.
4. I believe the Language codes and names are as defined by the
ISO 639-2 standard
5. Multiple files of the same type - users have devised a variety of solutions for this, searches calibre forum for 'additional files', 'supplementary files', 'extra formats' etc, should find some. Here's what I do
Calibre and Evernote The thread makes mention of some of the alternative solutions I used before I settled on that one.
BR