Well, since you're going to skirt around how the acceleration actually happens, I'm just going to counter with: The defender also has arbitrary time to defend. Couldn't you "store up" a defense over time somehow? Maybe have a counter-projectile orbiting really fast that can be brought out of orbit to hit the incoming one? Or, I know we don't have energy shields, but with the amount of assumption going on in this whole article, I may as well assume we do ...
The physics on storing kinetic energy like that just don't work.
One intriguing possibility I thought of is the possibility of creating a black hole[1] in the path of the projectile. I have no idea how the energy calculations work for creating a black hole, and what exactly happens to one if a large mass travelling at high speed hits it - I'm guessing if the black hole was small it would just slice the object in half. If it was big enough to "swallow" the object then I don't know what happens - if I'm reading [2] correctly its mass-energy increases, but what that means in practical terms I'm not sure.
Of course, the issue with this plan is before you had a fast moving massive object targeted at planet, but now you have a black hole nearby.
That is an oxymoron. You are either orbiting or going "really fast" but you can not be doing both. (In the context in question single-digit miles per second is not "really fast", it's just the cover charge.) You can pour arbitrary amounts of energy into a kinetic kill projectile, and there is no known reasonable way of defending against such a thing except to not be where it hits.
Just as people worry about nuclear weapons and biological weapons, we get planet-killing kinetic projectiles rather earlier than we get anything else useful from space. In fact if we put our minds to it, and were willing to be patient, we could probably do it today. The same tech we're looking at to divert existing planet killing asteroids can just as easily be turned to tweaking one of their existing orbits towards us instead of away.
