An FTS does not prevent 100t of debris of which a significant amount is designed to survive re-entry from impacting the surface. If you blow up 100t you still have 100t of debris, just in lots of bits (and honestly, still big chunks - an FTS does not atomise or even close - it punches a hole and then the vehicle collapses structurally)
All it really does is remove the explosive potential of the fuel.
So no, it's not designed to do this.
Once in orbit the FTS system is usually deactivated (safed).
It's designed to survive re-entry in a very particular, and repeatedly adjusted, orientation. It's unclear how much would survive re-entry in an uncontrolled tumble. But better safe than sorry.
Debris in orbits that nearly intersect the atmosphere are practically harmless. The issues arise when you have an orbit that will intersect other orbits a billion times before decay.
While the reality is very complicated, you can make a rough handwave model for orbital debris in a circular orbit: At 100km it lasts 1 orbit (90 minutes) at most, and every ~100km you add after that increases orbital lifespan by ~10x.
