#1
Walls are drawn with their center line specified by their X,Y-coordinates.
The discussion about whether or not this is the best way to draw walls, is as old as SH3D. Seen the fact the nobody ever had a better solution, that is still the way walls are drawn today.

#2
I'm assuming "abutting walls" means "touching walls".
Walls are connected at or between their X,Y-coordinates, ergo somewhere along their center line.
In SH3D that is standard practice.

H