Thank you Dan.

Your reply already helps a lot. I have removed the washer, and have tightened up the front element.
I have taken test shots, which I hope will be good enough to see any variations in curvature. Some shots done with the lens rotated out a fraction, and some with the front element screwed in completely without a washer. I also tried to compensate for the shift in focus due to the adjustments, so I'm hoping I did it correctly, and within usable margins. Time will tell.

As far as I know, the camera has not had the lens assembly completely disassembled. However, I'm not skilled enough to disassemble the Rollei and put it back together, so I couldn't tell you about the state of washers behind the shutter.

The scans will hopefully settle my curiosity as to how the position of the front element affects the curvature of field, if at all... If I cannot see any differences, then I need to either live with the curvature, or find myself another Rollei, which is near impossible down here... Its just a complete pain in the but to deal with the curvature for landscape shooting, especially if the problem isn't largely corrected for by stopping down to f/8.