Okay, i followed the advice of @Chopsaw, by removing the curved wall and instead drawing CAD arcs and lines to replace it. I converted the closed shaped to a polyline solid, then placed my arched cutout (the solid which i already created) into the exact spot where i wanted the subtraction. I'm really streamlining this explanation, because it took me plenty of trial-and-error to set everything correctly.
I then selected the wall shape, did the subtraction with the arched cutout, and fnally got it EXACTLY the way i wanted! To wit:
A suggestion for anyone trying to do complicated solid construction:
When doing subtractions, make sure the shape you're using extends beyond the faces of the main solid (that you're subtracting from), or it will leave a "face" on the side that wasnt subtracted all the way thru. This includes the wall sides as well as the ceilings and floor sides. I ended making a cut along the floor face of the wall solid, which caused a texture conflict between the room floor texture and the cutout texture on the bottom face. (The only way to solve that was to lower the wall so the leftover face was below the room floor — ugly and time-consuming!).
NathanielK's post in Need help with cutout in odd wall was marked as the answer