@jamesl1391 Hello James,
Fortunately @Krieghund already answered all of your questions. Just some remarks from me.
If you attack a zombie in you own territory, do they bite on an A or a D? ie are you defending your territory, or attacking the zombies?
The way I see this: The Zs are just conducting their battle phase, their throwing of the dice. And then the ‘D’ results can not be assigned. Just see this as a regular battle.
(@Krieghund , if I am wrong please correct me)
Can you hold a territory just with a plane in it, if it has a zombie - or do you also need a land unit there as well?
Very good question. Good you asked this so this could be clarified. Please mind that Zs can hit your - expensive! - plane in their next ‘Zs Attack’ phase.
If a single plane attacks a single zombie within a group of zombies (in a friendly territory held with other units), do all the zombies bite or just the one that the plane is attacking?
As far as I know in standard Axis and Allies games you never attack a specific unit…