How does breaking zombies work?

Solution 1:

• Do I deal less health damage to a zombie while it still has stamina?

The inverse - I don't believe zombies take less damage when they have stamina, but if they are knocked down (either as a result of having no stamina or via throwing/ramming/etc.), they take more damage.

• Will aiming at the zombie's head inflict more health damage, even when it's knocked down?

Yes, and you will occasionally get a time slowdown when this happens.

• Do certain types of weapons inflict more health damage compared to stamina damage, regardless of their base damage (for example, if I have a firearm and a blunt weapon with the same attack value, will the firearm inflict more health damage)?

It seems like the more Force a weapon has, the more damage it does to Stamina. I'm not sure how firearms stack up, though.