Hello,
the Planar/Xenotar was a remarkable improvement of field curvature against the Planar/Xenar. The problem of 120-film of not being flat in the camera is as old as this film. Carl Zeiss made investigations of this problem and found that the best flatness is immediately after transport. The film remains flat for about 15 minutes and than the curvature comes back. So it is best to transport only immediately before the exposure is made. The Rollei service had special non flat matted screens for testing the focus into which the average curvature of film had been grounded. The Rollei service also had exploded views of the lenses and how to center and adjust them. In this views you could see whether and where there should be a washer or not. Testing of focus was made with a so called autocollimator and they had very close specifications for measurement of the alignment of the front plate and the focus.
The best would be to give the camera to a real expert with appropriate equipment.