There is only one room in the Basement floor, and they are not isolated from each other.The Basement floor has a room inside it, which is called the room in the Basment.
The room in the Basement is within the Basement.Therefore to enter in the room in the Basement, one must be in the Basement, that was never questioned, and the theory never says otherwise.
The Basement is, obviously, the same as the Basement floor.Confirming the fact that being in the Basement is the same as being on that floor doesn’t disprove anything, there’s no need to be inside a room as long as you’re on the same floor.
Being in the room in the Basement is being inside the Basement.The reverse statement is not true, in other words, being in the Basement is not being in the room in the Basement.
Dalinar entered in the guest mansion while the group was entering it. Arya and Adolin are locked by Astrid inside the room in the Basement. Dalinar enters in the Basement using an entrance that was previously unlocked, or breaking down one himself. Dalinar kills Arya and Adolin while he was inside the Basement, naturally he never had to enter in the room in the Basement. Astrid unlocks the door with Shallan by her side, at that time, Dalinar was still inside the Basement, while Arya and Adolin were the only ones in the room in the Basement. Shallan or Astrid kill Dalinar while he was either inside the Basement or outside the guest mansion. Astrid kills Shallan and commits suicide.