Anyway... I am not convinced that curved backplanes are better than flat from an ergonomic standpoint. With a curved backplane, the further a row of keys is from the wrist, the more the key-pressing motion would be pressing away rather than down/towards you.
I agree.
I'm convinced curved plates are better than flat for one simple reason.[...] For the curved plate, the plane of the key surface is perpendicular to the travel of the key. You press in the direction the key is moving.
For the straight plate, the plane of the surface is angled off the perpendicular. You press in a way that's not in line with the key travel.
This seems incorrect to me. When you move your finger to find the new key, you’re extending your two distal joints outward/upward, but when you actually press the key down, the main motion is still to flex the whole finger at the first knuckle (metacarpal-phalangeal joint), and the direction of force is fairly close to straight down, not outward. (At least, that’s what it seems to me from observing my own hands’ movement.)
This is one of the things I find to be problematic on the Kinesis Advantage: many of the keys travel in an axis that is not the same as the axis of finger pressing, and as a result they are less uncomfortable to press than they would be if rotated to be more straight up and down. The Maltron has an easier time getting this closer to right, because it uses hand wiring instead of a curved PCB.
I do agree though that there might be an advantage in raising the further-back keys up higher than current keycap profiles permit. So perhaps making "steps" of sheet metal (Or CNC'd metal/wood/plastic) could be nicer to type on than current flat designs. Also, the proximal keys (bottom row or two, nearest the body) could probably be angled a little bit, as I think they tend to get pressed a bit toward the body as well as down.
In general, my expectation is that you get a much bigger ergonomic advantage from splitting the two hands apart, tenting them, and putting thumb keys in a comfortable spot, than you get from curving the plate the finger keys sit on. If you feel a pressing need for height differences in the finger section, I’d recommend switching to a column-staggered layout and adjusting the height between columns pressed by different fingers, e.g. raising the pinky keys up a bit. (But don’t try to accomplish this with a curved plate/pcb, or you’ll get the same problem again, where the pinky/index finger keys are pressed by unnaturally pushing sideways.)