Swarm of pebbles could safely deflect asteroids

[Fumesucker]

13. The density of any "cloud" of debris from multiple microimpacts is going to be thin..

That "cloud" also will be rapidly falling in density, any debris thrown up is going to continue in a very close approximation of a straight line with essentially unchanged velocity forever for all practical purposes.

With active maneuvering via light pressure there's no reason that a formation of spacecraft of any arbitrary scale down to bacteria size need disperse at all. Artificial smarts are fairly cheap and small now and getting cheaper and smaller all the time, a thimble sized self navigating spacecraft is by no means out of the realm of possibility and if you can make one you can make a million or a billion or however many are necessary.

This is very similar to the "Brilliant Pebbles" SDI concept but hitting a stealthed missile in flight with only very short notice is a far more difficult task than hitting an asteroid whose position and orbit has been known to a high degree of accuracy for a long time.