The staircase corpse DID become a zombieThe owner of the house tried to escape the house and got munched on by the ghouls. The corpse reanimated and wandered into the house. Then Cooper and Co. arrived, killed the ghoul, closed the doors and locked themselves in the cellar.
Why not go upstairsCooper was right that staying on the ground floor was a deathtrap, and Ben does have a point about only one way out from the basement. So why not get everyone upstairs, fill the stairway with stuff (beds, mattresses, chairs and other large furniture) that would make a barricade. No easy access from the ground for the generally clumsy ghouls and plenty of escape routes via windows if necessary.
The whole thing is in Barbara's headBarbara begins the movie visiting a graveyard. Early lines of dialogue, leading up to the first encounter with one of the dead, detail how Johnny used to scare her as a child in cemeteries. When he realizes she's still afraid, he starts teasing her again, claiming that the man they see is one of the monsters "coming to get" her. This triggers a latent psychological disorder in Barbara that locks her into a fantasy about the dead coming back to life and attacking people. This explains all the moments that make little to no sense, like the Coopers suddenly appearing after she and Ben have been alone in the house for an extended period. It's her psychosis expanding, perhaps incorporating people or character archetypes from her past in the struggle against the dead.
- Alternatively, Barbra was the one that hit her head on the grave instead of Johnny, but instead of the blow killing her, it knocks her out, and the movie is just one long, crazy nightmare for her while she is hospitalized.