brianco

The sensor is 43.8mm x 32.9mm. A 35mm negative is 36mm x 24mm.  36mm/43.8mm * 11,648 pixels = 9573. 
35mm mode just crops to the middle of the sensor in a section that's the size of a 35mm negative. This would be the area that "full frame" lenses are designed to cover. I assume it would be useful if you adapt a lens designed for "full frame" and don't want the vignetting that happens where the medium format sensor is larger than the lens was designed for.

I think you are saying you want to switch between custom settings and change that display setting (as opposed to a setting that affects the recorded image). As far as I know, the display settings persist across all dial positions (PASMC..). The strategy I use is to assign custom settings to modes of using the camera, i.e. landscape, portrait, street, etc. if there are different things I like to do in those modes, I decide where to put modifiers, like face detection, film simulation, etc. You have 3 more choices:  button, the Q menu, My Menu. If you need it instantly (face detection on/off, subject detection) I put it on a button. If I like to be able to see the state because  I change it often, I put it in the Q menu. I only use 8 and it's the things I change the most. MyMenu is where I put the next level of things I change, including most things I change from default so I can verify they are set correctly. 
tl;dr; Put it on a button if you change it frequently, or MyMenu.

I think the difference is more a matter of perception, of what looks acceptable. That's why the 100S has two different settings for the depth of field scale. On 6x9 film I rarely made prints larger than 16"x20" and distant backgrounds looked sharp. With the 100S I can see that the background is not sharp when I look at a file 100% on screen, but it looks perfectly fine in a 17"x22" print. My concern is that I want the files to hold up for larger print sizes, which was never a concern with 6x9 film.

It's a bad decision.
 
The x100s is my first Fuji camera and probably the last if Fuji refuses to provide firmware updates to "boost" sales of their newer camera.
 
I bought my x100s knowing that Fuji is the only company providing relevant updates to their camera lineup.