Everything you listed is correct except for the 2da file. The edge.2da is only used to define what appears beyond the area boundary. Edge tiles also have no walkmesh since they are inaccessible.
Regardless of if the tile is a TERRAIN or GROUP you need to do the following:
- Copy over the MDL, WOK, and any associated textures (might be a combination of DDS or TGA). don't forget to grab the minimap image as well.
- Add the tile to the SET file. Make sure that the boundaries defined for the tile in the SET file actually correspond to TERRAIN defined in the SET file. Also index the line [TILES=???] by +1 in the SET file.
IF the tile is being added as a terrain, you are done.
If adding as a group then you'll need to define the [GROUP] in the SET file and add it to the tileset ITP file (???palstd.itp). Also index the line [GROUPS=???] by +1 in the SET file.
SET Editor 0.85 is an excellent tool that streamlines all of this work for you and makes the process almost fool proof.
The NWN Toolbox has a ton of other useful tools as well.