[jdsully]

An elliptical orbit would require the perigee to be even closer to the earth further increasing drag.

It is possible for small fragments to potentially be moved to higher orbits (with counter balancing mass being moved lower) But the conservation of momentum prevents the vast majority of fragments from suffering this fate.

Baring exceptionally bad luck a collision in LEO would usually decrease the time before the orbit decays. Remember a debris field has more surface area than an intact satellite.