To back up your library, back up the folder (in your home folder) ~/Library/Application Support/Delicious Library/
To restore your library, restore that folder.
Borrowers show up as deleted because they have been removed from your Address Book. Usually they haven't really been removed, but their record ID has been changed by Sync Services. This is caused by synching in such a way that replaces the items on your machine with items from another source. You can do a merged sync to avoid these problems.
You can add any person in your Address Book into the borrowers, but you cannot permanently hide the borrowers field.
Neither can you clear the date of purchase. If you don't know the date, set it to something illogical, like January 1, 1900. At least then you'll know it isn't correct.
Many of these quirks are being addressed by architecture changes in Delicious Library 2.