Scratch my idea.
You are trying to restore a Library from just the files without using the metadata.db.
You are basically starting over... UNLESS you had used Modify EPUB (assumes all books are EPUB) and had it update the metadata/cover in the book (with what used to show in Calibre).
In that case, set the Import to read metadata (not use the filename) as the book should have all the current values (except date

, which is supplied by Calibre when it makes the DB entry)
2 Morals

1) Always make a FULL backup of the Library folder. If you goof, you just replace the folder wit the backup
2) Use a custom column for status and mostly leave Calibre columns for their original purpose (date=record creation timestamp)