Latest version of IWE brings even more features requested by the community. Most significant is interior placement editing. Finally, it's possible to delete, move or replace any model in interior rooms used by game. IDE exporter is also almost complete.
First of all, let's explain basics of MLO section of IDE files. Each MLO section in IDE files starts with subsection that contains interior name, object, room and portal count. Below that is a list of objects used by the interior, with names, local coordinates and other info. Next subsection contains names and other stuff related to rooms, including indexes of objects used in each of the rooms. Finally MLO section contains portals coordinates.
Each of the objects defined in first subsection of MLO has it's coordinates in relation to local interior 3D space. Coordinate (0,3,0) will always be 3 meters in front of interior world position. However, since GTA IV actually allows interiors to be rotated, all coordinates used in those interiors are rotated as well when loaded by game.
object in local space: 0, 3.0, 0
interior position: 300.0, 300.0, 0
object in translated space: -3, 0, 0