Re: One missile hit and it's all over.
Aside from the ship sinking, the other worry about a single hit is that a hit to a magazine will effectively disarm the ship by destroying most of its missiles.
The Zumwalt design deals with this in two ways: 1) The VLS missile pods are dispersed around the perimeter of the ship next to the hull, rather than being centralized and concentrated as in other USN ships, and 2) taking a page from modern tank ammunition storage design, the hull-side armor of the VLS pods is significantly thinner than the interior-side armor; any explosion in a pod should vent most of its force and projectiles outside the ship rather than towards the inhabited interior spaces.