With wide angle lenses the pancake design ensures parallelism between the lens plane and the film plane in a way that your rollei simply can't. Are the difficulties you're encountering really due to curvature of field or that the front and rear standards aren't perfectly parallel? Depth of focus with the 35mm @ f11 on the Aptus 75S @ the HFD, is only something like 0.325mm, so if the standards are a little askew that could easily account for what you are seeing. In my experience the 35mm is a stellar performer - though it does go soft fairly quickly towards the edge of the (admittedly large) IC.
That said, it's not all bad news for bellows cameras. I shoot the 28 on a Linhof Techno. It was originally quite a demanding lens to focus, but after a while I got to know its characteristics and now its fine - especially since, being such a wide lens, I usually shoot it at the HFD anyway. Because the Techno was designed for digital, the rear standard is fixed - machined from the same single piece of metal as the bed of the camera - which definitely helps with achieving parallelism. It has rear rise and fall geared in, and rear shift is achieved via the sliding back. Shifting the 28 is generally a no-no on large sensors with microlenses (e.g. IQ180), but on non-microlens DBs it's superb.
Yes, you could reuse your lenses in any of the cameras from Alpa, Arca and Cambo - but here's the kicker: you have to get those lenses remounted in proprietary mounts for each of the companies just listed ... and boy, oh boy, that ain't cheap - and has to be done at the factory. Take a look at some of the prices for Alpa mounted lenses - the mount is often almost as expensive as the lens! e.g. 28mm = $4152, in Alpa mount = $7832.
Jim
