The number of zombies you get is calculated at the beginning of each round. So, let's say you're at level 30 with 4 people. You should be getting 510 zombies. If someone drops during the round, you'll still have to fight off that same number of zombies for this round. However, for level 31 it will be calculated for 3 people, and you'd get only 369. So, the worst thing that can happen is someone dropping at the beginning of a round.
Whichever does the most damage. For everything except the Gut Shot, this is the head. Or, for some weapons (ray gun, grenade launcher, wunderwaffe) it doesn't matter.
Also, note that the zombie health varies on when the dog rounds occur. So there is a few levels of variance for when a weapon no longer is a one-shot kill. Mostly for things that happen in the later rounds, such as the PTRS.