What dangers present themselves when someone uses the grounding system as a neutral? I'm just looking for insight into the theories.
Unlike the European double-insulated 2-wire single-phase distribution system, the American single-phase system implements safety by using a 3-wire system consisting of a 'hot', a 'return' and a 'ground' wire. It takes just 4 milliamperes of current to flow from the 'hot' to the 'ground' to activate the ground-fault circuit interruption of a well configure 120 VAC power distribution circuit. Any more than 4 milliamperes results in sufficient current to cause cardiac arrest in large people.
If you are using the ground wire (green) as a neutral, not only do you have an unsafely configured AC power distribution circuit. But also, you do not know what "else" is between the green wire at the point you tap it and the point where the green wire is bonded to the return (usually at the panel). Almost all electrocutions that occur with swimmers swimming close to marinas occur because some electricity is returning via the green wire (faulty boat or marina circuit). DO NOT DO IT YOU COULD KILL SOMEONE IN YOUR HOME!!!
The neutral carries the unbalanced current. Depending on how the circuit is configured this unbalanced current can get very high. Plus years of electrical modifications to a homes wiring where things are added and tampered with adds to the neutral carrying more unbalanced current. So, what happens when someone haphazardly uses a ground as a neutral is they put all that hazardous current on potentially all the conductive metal items throughout the home, potentially creating dangerous shock hazards.
Neutrals come loose. Happens all the time.
Imagine a situation where a neutral wire fails to conduct well, due to looseness, wire damage or overload. What happens? The loads pull neutral "up" away from 0 volts. That's because the loads are "in series" with the weakness in the neutral wire. Draw it out in an electrical diagram, it will make sense.
If the neutral is cut, then the neutral will float up to 120V because it is connected to 120V through the load.
Now, if you have combined neutral and ground in some way, you are now putting 120V on every surface that's supposed to be grounded - including wall switch cover screws, appliance chassis, and even the service panel itself. So when you open the service panel door to check the breakers, ka-pow!
It doesn't really matter how you combine neutral and ground - whether you have ground and bootleg neutral, or have neutral and bootleg ground. Same issue either way.