I have a grid interface in my head for rebound, but am nowhere close to being able to code it yet.
I’ll do a quick brain dump, though, while I can still see it.
I’d want to keep the display as you have it, but for each ball, use the grid to establish starting position within that space (press and hold a button), direction (tap a second button before releasing the first – the ball is propelled at the angle between those) and speed/velocity (how much time passed between establishing placement and direction? Take the relative distance between your two buttons, and divide it by this span of time…)
From there, the initial grid coordinates become a reference to that particular ball, so you can quickly select it, adjust its trajectory, mute or delete it, etc.
(This does need a screen. I don’t think it makes sense to display your animation on the actual grid, as much as I’d want to.)
Hopefully, these thoughts prove more inspiring than confusing, but at this hour, that’s hard to judge.