Thank you for your post.
The way that you've shown in the image is the way to currently remove the triangular chunk where the walls meet by connecting the walls together as a 'butt joint'.
Currently the only way to change the wall top & side materials is to go into the walls and manually apply the materials. Please note if you do this that any manual edits will be removed if/when the wall(s) are redrawn or edited so we recommend to only do manual edits at the end of the job or if no further edits will be made to the wall or other objects.
In the next version (which will be available in the next week or two) the top & ends of walls will have the same material applied to them as the outer material (e.g. if a red brick is on the outside of the wall then the top & ends of that skin will also have a red brick material.