A rubber band is a spring. Springs arent simple machines, they store energy then release it where as a simple machine conserves energy but multiplies force.
Ideally, yes. The energy that goes into a simple machine should come out. The job of a simple machine being to exchange force for displacement (or vice versa). In reality, some of the energy put into a simple machine is lost to friction. As a result we have to differentiate between Ideal and Actual Mechanical Advantage.
