t-bear, 2000i does view ADT Objects. There is an extensive library in 2000i that you can install 3d objects. Drop in a refr, a desk, a toilet, it's great, takes seconds where drawing the stuff takes minutes more.
When you use flatten with ADT objects, it takes the object from the base point and pushes it to the zero plane. You still have the object, it's just at the zero plane. So if you have windows in a wall, they will all be on the floor.
As was mentioned, to my knowledge at the moment, (because I don't use 2D stuff much at all if I don't have to) the best way to make a 3D into a 2D is to 'saveas' a previous format, R14 for example. The problem is in creation of the file, some lines just don't connect very well, although after the conversion, open the file, use the flatten, purge and then save it as a standard dwg format of your choice and you should have the 2D file.