Hello Mr. T.S. there are at least a couple of debates about this in other parts of this site and it exists on the wishlists in many instances. As far as i know Archicad doesn`t correctly project trimmed or SEOed walls on plan, which is a downer. But fear not! a lot of people are constantly asking Graphisoft to fix this.
Anyway, if you require to project it correctly, my suggestion would be to copy the trimmed element (make sure the trim is mantained in the copy or trim it again) and convert it to morph, that way, it will project correctly even if you change the section plane height and also the sloped part of your wall will show as uncut if its below the cutting plane.
I would put the original walls on a hidden layer in case you further need to edit the wall down the line. Hope it helps!