Are your operators set up with substitute operators on the work tab? If they are, you could leverage what you have there to determine who to notify. And if you are using the Operator is available to receive work checkbox, you can leverage that to know when the operator it out of the office.
OOTB Case management handles the substitute operator while routing. As already mentioned substitute operator can be mentioned in the operator rule itself and while routing Case management takes care of routing work to substitute operator if current operator is not available.
You can apply the same concept in your notification activity where you can check for substitute operator and send the notification.
See the attached screenshot for more clarity and let us know if you need more information.