yes the 555 will work from 3v to 18 v I think. And yes you are correct about the voltage. Most logic circuits run on 5 volts. But it would be a safe thing to do, to turn your monitor on, and put a volt meter across the contacts of your button to see what voltage the switch runs at. Also you would just use a power off circuit to trigger pin 2, the 555 would send another pulse to switch the screen input when you leave reverse.