Here is a silly theory
The living group and Dalinar entered the guest mansion. Dalinar may have used the same entrance or another entrance to the guest mansion, it matters not. Arya and Adolin had the basement keys on their person. They got into a scuffle and killed each other win a gun outside the basement. Astrid and Shallin found the bodies. Astrid, through or otherwise placed their bodies in the basement and locked the door with her master key. Afterwards, Dalinar went to the same room as Astrid and Shallin on the first floor. Astrid killed Shallin and Dalinar. She then killed herself