There's one problem with your math.

Grains of sand surviving re-entry. Even at near light speed, a grain of sand would not survive that intense of a temperature. The fact there are MILLIONS of grains means nothing as each would simply vaporize entering atmo. Sucks, but that's logic for you. All you would do to that planet is give them one HELL of an impressive light show. Granted, it'd scare them like mad. But really, it'd do no damage to a planet.

Planet = Unharmed.
Ships in vacuum = SCREWED.