I'm pretty sure there's no way to tell what individual particles are doing, so you can't tell when one leaves the layout.
But changing the timescale can be done based on how long you'd expect it to take to fill the screen, so you can just set timescale to 20 at the start of the layout, then to 1 when timer = 15 seconds, or something like that.
Regarding the effects - yeah, I see what you mean! I'd never noticed that before. It only seems to happen with some effects: "Outline" or "Greyscale" apply correctly, but "Soften" works for one instance of the particle object and breaks the others.