A Zombie could definitely solo 'mellow' 5.8. To be safe I think you'd want to find a tech 5.10+ or at the very least an offwidth 5.9 (lets see a zombie try to hand stack up a crack).
Devils tower would be a prime location.
Living in Jackson, life would probably carry on as usual. There's hardly any people in Wyoming and everyone is already armed to the teeth. Not to mention, grizzly bears love to eat zombies.
If the town did fall, I would post up at granite hot springs. The caretakers building is already 100% off the grid and its way the fuck in the middle of nowhere.