So much for suggesting and thinking out loud!
"I'm thinking that if I change the status of 800 to 700 or simply delete those three records from the mdl_backup_controllers table, the process should run successfully again. What do you think?"
Don't think that will do anything. I have seen many course backups reach status of 900 or 1000 to finish successfully. Think that table isn't really controlling anything but simply reporting.
The way I thought moodle cron/task worked ... if a task begun, a lock is placed on not only it but all things related to that task so that no other, what might conflicting process, can until it's finished.
Since moodle uses moodledata/temp/backup/ as the area to build resources for the building of a .mbz file there is one task for cleaning up temp file areas of which temp/backup/ would be one.
Since you say it takes a day or more to finish autobackups, can you set anything seen in task for cleaning up files/tables that you could set to run @ a time just a little after autobackups?
Did you read Alian's thread? Thought he described how he offloaded autobackups to another server which wasn't a full clone of production site's code, but just enough to run the cron/task.
'SoS', Ken