I think (hope) I can answer these.
1. As far as I can tell, the deletion of modules and the levels contained within the modules is managed outside of the editor. Find the files in your installation folder and smack that delete key.
They'll be found at \\*installed location*\Divinity - Original Sin\Data\Mods\*MODULENAME*\Levels\*LEVELNAME*
2. You need to highlight the wall and use mousewheel to scroll through the prefabs. Different wall-types will have different variations of walltype, so you may want to try out a few until you find one which gives you several options. Remember also that you can right click the wall -> split wall to turn a long section into two separate short sections -- this can help in increasing your options. See this video tutorial,
https://www.youtube.com/watch?v=rsSf8wI1Qd03. Flat floor textures are handled the same way as placing the initial 'base terrain'. Go to terrain editor, highlight your terrain01 (or whatever it's called and hit the green plus sign at the bottom. You'll get a pop-up -- select the resource button on this pop-up. In the resource editor, right click the blue ball icon to filter out non-floortexture resources, highlight 'Assets' on the file tree and search 'floor'. You should get several usable resources. highlight the one you want and hit the 'add' arrow key in the pop-up. If you now reselect 'Terrain01' in terrain editor, you should see your floor texture and can begin painting.
Example:
![[Linked Image]](http://i.minus.com/ibip75nQ6B24bm.png)
Also, see this video tutorial (skip to 3:20 if in a rush),
https://www.youtube.com/watch?v=wv93lt_wXbQ