The map editor is any wolfenstein map editor. There's a script that compresses it into the format the game uses (it just merges the layers).
For the other episodes, would need to have the sprites and behaviour for the other enemies, plus the textures. It's doable but I don't want to do it :)
I'm not sure how to read the mouse. Plus space is extremely tight (I think I have 50 bytes free total, in two different sections).