As for Automatic Defrost, normally it runs for about 20 minutes, long enough to melt ice and allow it to drain into the pan beneath the fridge/freezer. And the reason for the thermal inertia (not a true effect) is because it takes so much change in temperature for a simple thermostat to trip from one state to another.
If you want more precise control over temperature then you should get a fridge/freezer with electronic control.
As for the simple thermostat, it's composed of a bi-metalic strip, two strips of metal that are dissimilar (different) from each other. Both have thermal expansion coefficients which dictate how big it will get when it warms up. One metal expands more than the other, thus bending the strip. At some point it snaps over and causes an electrical switch to turn on, and strip away heat from inside the fridge/freezer.
The "Thermal Inertia" I mentioned, though there's no such thing, is an effect of the differences of temperature that are required to trigger the switch on or off. And bi-metalic strips are not precision devices. It may take 40 degrees F to trigger the compressor one time and may trigger again the next time at 43 degrees. Or 37 degrees. Some of the variables come into play based on how fast the temperature is changing. If the door is opened and cause a sudden change in temperature you can see spikes in the temperature before the bi-metalic strip kicks over. Hence, you get what appears to be thermal inertia.
Electronic temperature sensing is also NOT an instant detection thing. Even an electronic temp sensor needs to react to sudden changes in temperature. Still, they can be programmed to operate at a much tighter range. The compressor may kick on at 35 degrees and back off at 33 degrees. And that would be considered some pretty tight tolerances for temperature control. Likely it would lead to premature failure of the compressor because of excessive cycling on and off.
As for the "Unhealthful" section of your question, frozen food is not subjected to "unhealthful" conditions unless the food is allowed to thaw.
As for the "Tolerances" at which fridges operate, that depends on the manufacturer. But rest assured, nobody wants to put a product into your house that makes people sick. So whatever safe level of temperature fluctuation may be, fridge manufacturers stay WELL AWAY from the danger zone.
So if you're worried about your defrost cycle spoiling food then go buy a regular refrigerator. One that is NOT "Frost Free". Also, frost free fridges tend to dry out the food. Which is why the freezer in my basement is NOT a frost free type. Every six to nine months I have to (or elect to) defrost it manually. We'll put everything into coolers and open the door. Place a low heat hair dryer pointing into the box and melt all the ice. We hasten the event by using plastic scrapers to help chip away and remove fallen chunks of ice rather than entirely melting them.
Hope this helped.