metadata.db is a 1 to 1 match to the books and folders, within the library.
In some cases, an older DB might recover
most. All depends on what/how much happened in between
Whatever you do, DO NOT mix backup libraries together. Recovery will become a real nightmare.
If need be, recover each into individual libraries, then when each has been made error free, use Calibre to merge the needed books.
Find Duplicates PI can compare 2 libraries, then you can make determinations from the remains