One of the reasons I added the two zener diodes to the design was that the P-MOSFETs I had on hand had only an 8 volt maximum Vgs and I needed to run the system at 12 volts.

You can get better transistors, though. The DMP3099L-7‎ is an excellent candidate. It has a maximum Vgs of 20 volts, a maximum Vds of 30 volts and a maximum drain current of almost 3 amps at 70ºC. If you use that part, then you can leave out D2 and D3 and use 0Ω for R2 (though it does no harm to leave it at 10kΩ).

EDIT: In addition to better transistors, I've also discovered a SOT23-5 5 volt LDO that has an ultra wide input range. It was a little extravagant to use the D2PAK packaged 7805 for this job, given that the ATTiny9 draws maybe 1 mA total. Even if it were 1 and you were running the whole thing on the maximum input voltage of 20 volts, that's a dissipation of less than 15 mW. That's 3ºC above ambient (given the 235 ºC/W to-ambient dissipation rating for the chip), which is not a big deal (and that's the worst case).