MCs group were clearing zombies out by the hundreds in enclosed spaces before so cleanup isn't an issue for them. Zombies don't attack Mc because they're distracted with other humans that were right in front of them. He crashed back first so unlikely to incapacitate himself, and he can always just drive away.
Zombies getting to hostages... Well they were banking on the enemy being smart enough to lock down the entrance,. If they weren't then... Well there's always the next group, not like they lost much doing this, anyways, and they'll get a huge pile of food out of it regardless.