Falloff now just goes from 0 to 1, so adjust that how you need.
The jittery thing might be because you use too low of a resolution in your game. I do not know how you setup your camera view and application surface/window, etc. Look at the example project included and see it is not jittery like that.
You may also need to change the shadow map size to better match your resolution.
For your light objects, I cannot see your code. This sounds like you are doing something incorrect with your light switch object. Do you have a solid understanding of the difference between an object and an instance?