Genome -
I think I am ready to ask you some questions (please forgive my ignorance).
I did not notice you had any. It's probably me that has little knowledge of your subject.
1. Why turn on the M2 mosfet with a current limited source (why can't I get rid of Q2 and connect the Q1 emitter to gnd?)?
Because you are dealing with an automotive environment then effectively voltage levels are not guaranteed. That might raise a question about what happens when your battery is heavily loaded.. This will be my inexperience.
Otherwise..
Depending on the device you choose then Mosfets need a certain gate voltage to fully turn them on. There is also a maximum voltage you can apply to the gate before you blow them up. That's why the Zener Diode is there.
A 12 Volt battery is probably good to go but in order to fully enhance the Mosfet you will need all of those volts which means you can't place a series resistor in the collector of Q1. It would reduce the available drive current to what might be effectively zero.
When 'nasty' things happen, load dump or something, you may well end up with 'Vbatt', at your circuit location well above the expected 12V at which point/time you would apply it across the gate of the Mosfet and kill it. Either that or if the driver was not current limited it would get stuffed via the Zener Diode.
Since the driver is current limited then under over voltage conditions Q1 will 'take up the slack' or difference between the 15V Zener and any over voltage.
Otherwise it is a 'nice' thing because it guarantees you that drive current to the Mosfet. I'll admit it is not large but it should be sufficient to switch the device quickly enough. That will be my inexperience again.
2. Under what scenario is the zener protecting the gate of M2?
As suggested, given an automotive environment, then voltages are not guaranteed and there is the real chance that you may end up exceeding the Vgs, Gate to Source, rating of the device. The Zener diode, in conjunction with the current limited drive, will... hopefully prevent that from happening.
Thank you
p.s. I have just installed LTspice and was wondering where you obtained the battery component. I can't seem to find it under components.
As far as I know the Battery Component is nothing special. It is just a graphical representation of a voltage source. Otherwise it lives under 'misc'.
Genome