I'm afraid we'll need more info for both of your problems.
Because of floating point errors, you'll never have infinitely bouncing balls with a physics engine unless you listen to collision event and apply forces manually each time so you can be sure that it won't lose momentum and override what the engine does. I think this is the best solution. But maybe you should explain what your app looks like to know if that is an acceptable solution for you.
Are you using starling? to what target is your mouse event listener registered?