Lazlo, in this case the 2D is important (I already had the 3D anyway)
Jared, yes you are correct that the priorities were not right. Fixing them did improve things but did not correct things completely.
Playing with this some more and trying Snap's thought to use a simpler angled walls has not resolved things yet either. It appears that if the vertical side walls are shorter in length than the projection of the angled buttress wall (keeping these two elements completely separate from the rest of the model to avoid any other complications) then they fail to clean up as shown in my screen shots. If the vertical side walls are just slightly longer than the sloped wall, then the clean up is appropriate. Now I just have to play with the model to see if I can make that work with everything else.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K