"Grain Solver or the Vellum one?"
if you have a good GPU go for vellum, if not, grains is still a solid option.
"Also, after all, I need to use a custom Force to "Explode" some Grains in the air, because it seems impossible to happen with just the ball collision."
True, this is the way
You need a mid-res grain simulation with decent motion + upresing on top of it + additional particles simulation.