As a result of weight painting vertex group(s) are created; if you assign something to new vertex group there is weight slider under (default=1) - change value, assign again and you get this weight. So it’s the same in general.
Moving to 1st ;).
Particles as a light ‘cloud’… While light ‘fills area’ (what you can see if there are fine particles as a dust or mist in the air) in ‘real’, no way it works in Blender. You would need several millions of particles, even more emitted light rays and a lot of waiting time at hands.
Since that is an animation, even Cycles volume lights would make significant delays in production ;). You could try, it does look something like this:
and consists of scaled cube where material is to the Volume input attached Emitter node. One simple Emitting plane hidden into the box on top.
Rather make it so that part of the light emitter is visible in camera, assign material index and render on separate render pass, then compozit some glow. Will move the project into reasonable time frame.