MOSFET Gate-Source protection question
Your interpretation for the reason why Z-diodes or resistors are used is corrent, in my opinion. 

When the driver leaves the MOSFET's gate floating, you want at least something to give you a defined voltage, and a single resistor is an often-used solution. You choose the resistor's value as a trade off between a "good" low with a floating driver output (i.e. a small value for the resistor) and low losses while the driver has a high signal while turning on the MOSFET (i.e. a large value for the resistor).


A series combination of a diode and a resistor is just another trade-off, and one I have hardly ever seen. It prevents excessive currents should the diode ever start to conduct, while it "just somewhat" limits the gate-to-sorce voltage in such events. Also, a resistor is always a dampening component in any circuit prone to oscillations; it may have been designed in to prevent ringing once the diode conducts, but I'm really just guessing here and have never used such a snubber myself; usually, you want the snubber in the drain-to-source path, because this is where oscillations are usually caused. Putting it in the gate path will likely only limit effects that originated somwhere around the drain.
A (bidirectionial, like in your picture) TVS is just two zeners. Some diodes marketed as TVS have higher peak power handling capabilities or are designed for really fast breakdown. The anti-serial-connection of two diodes goes along with the specification of VGS, max&nbsp;=&nbsp;&pm;&nbsp;(some&nbsp;voltage). In words: Negative gate voltages are OK for the MOSFET, and sometimes, a gate driver will provide a negative voltage to turn off the MOSFET, either for a "better" off-state, or because VGS, th is quite low, or because your FET is a normally-on type (although that's not mainstream for the application of motor drives).
Yes, that's basically it. Just for completeness, one should as well add the obvious VZ&nbsp;>&nbsp;Vdriver, ON, because otherwise, the driver will just dissipate power into the protection diode.
(One more answer to one more question ;-) Sure, this sounds like a good idea and I believe I've seen it many times.