Jpd.
Never thought about fine tuning the focus like you have, and it also looks like you are talking about very small amounts of adjustments in focus.
One advantage with a Selfix 820 is the double exposure prevention thingy - unless you use a cable release to fire the shutter that is. This device means its easy to first fold out the camera, then wind on the film from the previous shot, because the shutter wont fire again untill the film is wound on, and assuming the film pressure plate is doing its job, there is no reason why the film, at this stage should not be as flat as it gets.
I suppose different films do actually have different qualities when it comes to lying flat, but for me the most important improvement in that area was to check out how effective the pressure plate was, and in my Selfix 820 it was crap! - not enough pressure, and it missed in 2 opposing corners of the frame. I couldnt see how to take it off to improve the springiness, so I made it thicker, and without being able to see what it actually does to a loaded film, I now assume it keeps the film pressed against the guides.
I know modern gear is generally dead good, but some of these old folders, (and the folks who use them know good ones and bad) are pretty good, and cheap, though they do seem to require though a fair amount of tinkering around with.