Re: Kessler syndrome
Kessler Syndrome sounds realistic, but it's likely a too-simple model.
One big issue has already been mentioned: total momentum is conserved, but a big part of the kinetic energy is changed to heat. So only a small fraction of the debris will have the kinetic energy to remain in a stable orbit. The vast majority of collision debris will be put into highly elliptical orbits. An orbit that isn't nearly circular will dip into the atmosphere at perigee, with fiery results.
A second effect is more important: small objects quickly drop out of low earth orbit. Their cross-section area is high in relation to their mass. Collisions with stray molecules (too thin to be "atmosphere" or call it "drag") causes a rapid decay in the orbit. Again, ultimately with fiery results.