Oh, I was acting under the assumption that it was coming from somewhere besides earth (eg. aliens, some other human colony).
Because I can't think of any reason that you would: Send if away from earth, stop (gravity slinging wouldn't work at anything near .1 c), turn around, then crash back into it (especially when you already have enough weapons on board to kill 95% of the earths population.
In order to use something like this as a efficent weapon you would want to fire it from far enough away that the acceleration isn't easily detectable (+10 light years would probably be enough).
Otherwise you would be open to counterattack very easily, and its primary useage as a weapon (the complete stealth until a few days left) would be gone.
2. You can project orbits with a fairly good margin. It's speed makes manoevring with it near impossible. So it won't be that hard to shoot down things with it.
Pretty good margins isn't perfectly, especially since it could still fire up its engine and subtly change its trajectory (eg, by accellerating by a tenth of a mile per second, by changing the angle by a thousandth of a degree).
And even if we destroyed it before it could hit, once it got close (but not close enough for us to destroy it) it could fire out all its nuclear weapons for a barrage of super hard (since detection would be messed up due to the debris cloud of the ship) to detect nukes, which while not planet ending, could kill hundreds of millions.