In what preference file does Time Machine store the backup Volume name?

"Time Machine's main preferences can be found at /Library/Preferences/com.apple.TimeMachine.plist (which you listed - but the name is stored there)

The Backup hard drive is actually stored as an data blob(1) under the key BackupAlias. Property List Editor and most text editors (eg. TextMate, SubEthaEdit, BBEdit, etc.) won't show the information after converting the preference list from binary to xml using plutil. However you can use PlistBuddy (available in /usr/libexec/ which is not in the default path) to give you the name of the drive (with a bunch of repeated information):

/usr/libexec/PlistBuddy -c 'Print :BackupAlias' /Library/Preferences/com.apple.TimeMachine.plist

1 - This is the base64 decoded form of a _CFULRAlias to be exact."