Gotta keep that trillion dollar Ponzi scheme floating!
In case you're being serious, Gravity is a notoriously unrealistic illustration of orbital debris.
Kessler cascades are real. Kessler cascades taking out wide ranges of orbits in a matter of hours is not.
The intuitive fix is to understand that the volume of space around Earth is enormous–LEO is about 200x more voluminous than commercial airspace. It's entirely possible to trash a single orbit. But that would take months to years to manifest. And in LEO, it would tend to only last years.
To my knowledge, there is no solution to efficiently denying orbits by purposely triggering Kessler cascades. If it's cheaper for a military to shoot down individual satellites rather than trigger self-replicating cascades, it's not going to happen accidentally.
(Exception: GEO. That's a single orbit. It's also high up, which means it clears very slowly. It would be entirely possible for us to screw up geostationary orbit for generations.)
It won't massively change the orbit (e.g. it won't either come down or escape to Solar orbit for tens of thousands of years), but in will change the orbit inclination quite a bit and also the period IIRC. And given how thin the usable GEO belt is (so that the GEOM comsat does not move from the ground PoV, it should be basically self cleaning - any dead sat or debris will quite quickly exit the thin GEO belt.
You might just need to watch for the cloud of stuff around it when maneuvering or if it ends up intersecting once in a while.
Countably, yes. Broadly, no–the energies prohibit it. (Plane changes are expensive.)
If orbital mechanics permitted such cascades, solar systems wouldn't exist. Everything would be scattered clouds of constantly-colliding debris–not a neat plane.
Not to mention having all those space stations in the same orbit about 200 meters apart.