The newly released section drawing of the tomb elements discovered so far in Amphipolis is putting the lay-out in a clearer perspective, I think.
From an article published in the Archeology News Network, it seems that the narrow door (96cm wide) in the third chamber is off-centered and shifts to the left, hence the assumption that this door is leading to a staircase on a lower level. My personal opinion is that it may open into a narrow corridor to the next chamber as I don’t see how such a narrow opening could open into the main chamber.
As to the hole in the upper right corner of the fourth wall, Mrs. Katerina Peristeri is convinced that those stones were not manually removed but were probably dislodged by an earthquake. This is the main reason why she believes that the tomb has not been looted.