Unable to re-enable Spotlight on Volume
There are typically user immutable flags set on Time Machine backup volumes so that even root can't directly chown or chmod. You can look at these with:

ls -laeO /Volumes/*
ls -laeO /Volumes/Backup/Backups.backupdb/


Mac:~ me$ ls -laeO /Volumes/Backup/Backups.backupdb/
total 0
drwxr-xr-x@  7 root  wheel  -  238 Mar  3 04:40 .
 0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown
drwxrwxr-x  20 root  wheel  -  748 Mar 25 12:29 ..
drwxr-xr-x   3 root  wheel  -  102 Mar 27 12:04 .RecoverySets
drwx------   7 root  wheel  -  238 Mar  6 10:47 .spotlight_repair
drwx------   2 root  wheel  -   68 Mar 27 14:28 .spotlight_temp
drwxr-xr-x@ 40 root  wheel  - 1360 Mar 27 18:31 Mac
 0: group:everyone deny add_file,delete,add_subdirectory,delete_child,writeattr,writeextattr,chown


Cross reference with diskutil list to double check your mount points and volume names. My experience is that since Time Machine requires spotlight to make hard links is that if you tamper with the stores where those databases are resident on the filesystem you can't put "humpty dumpty" back together again and should archive off the snapshot backups you need using Finder/rsync or by making a dmg of those instants and then wiping the drive to start with new slate of .dotfiles and .dotdirectories with associated stores.

Clearly, someone with internal knowledge of how Time Machine and spotlight self check might step up and explain things, but I've not seen this level of detail publicly documented by Apple so far. I'd start at http://pondini.org/OSX/Home.html if you haven't checked there for any community knowledge on the innards of Time Machine + Spotlight