@thrasher1
(1) The presence of zombies does not itself make a territory hostile - it’s about who controls it. If you control a territory that contains zombies, it’s just like a territory you control that doesn’t contain zombies, with one exception: you are allowed to in effect treat it as a hostile territory if it contains zombies. This means you can make a combat movement into it and/or attack it, and also that units may combat move out of it if you’re attacking it. Other than that, the normal rules of combat movement, combat, and noncombat movement apply.
In your example, the units in Western US could legally do all of the things you mentioned. If they attack the zombies in Western US, any units not attacking would need to move out in combat movement, to either hostile or friendly territories. The only way to move out in noncombat movement would be to not attack the zombies.
(2) I can’t find a card that gives you something “if you kill at least 3 zombies”. You must be referring to “Research Mission”, which grants you a technology “if at least 3 zombies are destroyed”. In this case, it doesn’t matter who destroys them, and it doesn’t matter whether or how many new zombies are created.