We have the Sunpass and use it for rental cars. We register the car as soon as we can, then when we leave, we remove it so that if someone else rents the car and has a violation, it won't be on us. :_
Yes, you're supposed to but I doubt very many people do. It will work regardless of what car it is in. Just make sure you hold it up to the windshield as you pass through toll sensors. And if the rental car already has a SunPass in the little metal box, definitely don't open the sliding door on that box or you'll be double-charged.
In many places, yes, but not everywhere. I-75 to I-4 saves you $.50 and if you were to go from Orlando to Miami, the savings is over $4. But at some exits there are no savings compared to cash. And at toll plazas marked with Orlando's E-PASS logo, there are no savings but your SunPass is valid.