#1. The core of an iTunes library is a file called iTunes Library.itl. This is the database that records what files have been added to the library, and also any information that isn't stored in the tags of each track. The tags include things like track number, name, artist, album and such. What isn't stored in the tags are properties that may change during playback or are only used by the iTunes library, such as rating, play count, date added, playlist membership, etc. You can rebuild some aspects of the library from the tracks alone, but if you want to backup the full state of the library you need the iTunes Library.itl file as well as the media. Any way that you backup is better than nothing, but I recommend SyncToy because it can scan the source and backup copies of a library looking for changes, and tell you what those changes are before it mirrors them to the backup set. This saves redundantly copying files that haven't altered, and gives you a chance to stop and undo anything that looks wrong, like say deleting a whole bunch of tracks that you don't want deleted. It is too late now, but a backup of the .itl file might have made it easier to reverse the mislinked tracks. You may have an old backup of the .itl file from a Previous iTunes upgrade. See Empty/corrupt iTunes library after upgrade/crash for a post that describes the way to use on of these files. (Don't do anything with that link for now, I'm providing it for reference.)
What is the date of the most recent library file in C:\Users\Mark\Music\iTunes\Previous iTunes Libraries?
#2. Please. It might help me understand what happened. It would also be useful to the locations of the duplicates.
#4. Windows Explorer AKA File Explorer on Windows 10 is the tool that you use to browse the folders and files on your computer. For the track you mention the expected path would be:
C:\Users\Mark\Music\iTunes\iTunes Music\Compilations\International Superhits\05 Basket Case.m4a
From past experience possible alternatives for that track would be
C:\Users\Mark\Music\iTunes\iTunes Music\Music\Compilations\International Superhits\05 Basket Case.m4a C:\Users\Mark\Music\iTunes\iTunes Music\Music\Green Day\International Superhits\05 Basket Case.m4a
C:\Users\Mark\Music\iTunes\iTunes Music\Green Day\International Superhits\05 Basket Case.m4a C:\Users\Mark\Music\iTunes\iTunes Media\Music\Compilations\International Superhits\05 Basket Case.m4a
C:\Users\Mark\Music\iTunes\iTunes Media\Music\Green Day\International Superhits\05 Basket Case.m4a
but only you can find the true location, either by exploring or searching your computer. You are looking for the file 05 Basket Case.m4a and whatever folder it is stored in so you can create the full path similar to one of the above.
It is worth noting that your library has the old name for the media folder iTunes Music. In a modern library this would be called iTunes Media. You may have both folders inside the iTunes folder. Newer libraries also put the compilations and artist folders inside another subfolder called Music to keep the top level of the media folder tidy. In older versions the artist folders were at the same level as Audiobooks, Movies, Podcasts, TV Shows, etc. I suspect the original issue was cause by iTunes mistakenly reorganizing some of your content into a newer layout, but not updating the saved paths, or vice versa.
Please find 05 Basket Case.m4a and tell me where it is.
tt2