How do I force Spotlight to stop corrupting my external hard drive and destroying my time machine backups?
Spotlight is mandatory for Time Machine volumes. You cannot fully disable it other than disabling Spotlight completely. 


  If you add a Time Machine backup disk to the privacy list, you will continue to see messages that Spotlight is indexing your backup disk. This indexing is necessary for Time Machine to function properly and can’t be disabled. Spotlight does exclude from searches any items you store on your backup disk that are not part of a Time Machine backup.


As for disabling Spotlight/ejecting your drive safely, there are various solutions here, where the necessary command appears to be 

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.metadata.mds.plist