Ic-racer, thank you for your comment.
The issue of film not lying flat had crossed my mind. However, the Rollei infrared film, and also some of the other Rollei films, are films that have the least curl I know of. Both vertical and horizontal. The horizontal curl is often such that the emulsion would, if anything, tend to curl in the opposite direction to the lens, pushing the centre into the backing paper. I've had a warping issue once with Tri-X, but thats the only time I observed the effects of in-camera film warping. My other Rollei doesn't display any strange or strong filed curvature issues. Somehow, I tend to think it can't just be the film. I have shot many different brands, PET or triacetate, thick and thin emulsions... on the 2.8D, the behaviour has always been the same.
Regarding what you said about the front element position; Do I understand you correctly, that moving the element away from the shutter, that the perimeter will focus further towards infinity while the centre will largely be unchanged? ... In my case, mitigating the curvature I'm experiencing? I don't understand enough about optics to imagine the result of moving one element or group of a lens.