It finished on its own after about 48 hours, so we are out of the woods for this week.
But we would like to know, for future knowledge (and ongoing anxiety control) - would it be safe to kill dbms_wm.deletesavepoint?
Yes, it would be safe to end the execution of DeleteSavepoint. This will cause the open transaction to roll back. I would recommend to make sure not to stop the rollback. Let that part of the process finish.