I have a float switch on the sump as a safe second.
How did you set that-up... I like that idea a lot. Right now, I have my return pump elevated so that it is 2 inches below the water surface of the sump. If my overflow stopped working, my return pump can't continue to pump water and overflow the display tank... the pump would start to run dry before that would happen. So far, that has worked fine, however I worry about what could happen if the pump did have to run dry for an extended time (like while I was at work).