Sheet+wall in same location gives a flickering sheet. I know that, but I don't want to move the sheet or make it fit in a triangle shape, cuz that always gets messed up when it's not in line with editor's snap.

Any way to fix it without, like, putting the sheet 1UU further?

Well, the sheet is sticking out in the open a bit, so a part of it, the part that's flickering, shouldn't be visible. So doing that will make the whole sheet visible. Here, this map should show it:
http://www.multiupload.com/ANHBMY9P74
I can't make it fit cuz it's not in line with the vertex snap so it will get messed up if I try it.

Make a 16 thickness cube of similar dimensions to the sheet (sheet extruded 16uu depth-wise). Add it in the same place as the sheet as a semisolid. Flag all surfaces except the one you want to see as invisible, and the one you want to see as 2-sided.