Nice!
Why not just go full character mode?
The smooth motion is nice, but a bit of overkill, also it adds complexity to the collision code.
I suspect this can be done using a variation of Bresenham's line algorithm for the trajectories (keeping delta X and delta Y for each ball) and avoid most/all of the complex trig and math. Just addition and subtraction and a few sign changes.
The delta can be kept at a higher resolution than the grid (i.e. 16x so a mask can be used instead of division), so you can fudge the collision with small delta changes on impact using the SID as a RNG.