All of the wars and relations are stored in the 2,000-some-odd text files, namely the wars and country files. They can, of course, be edited.
If you select a specific date, it does not recommend countries for you. The bookmarks are just to jump to interesting events and suggest interesting countries.
I can assume you can have duplicate entries. The only restriction I've seen in the length of strings in the txt files; that's why my descriptions are kind of short. You can, however, make them longer in the text.csv (the file that contains all of the game text translated into several languages), but that requires another file to upload.