I have read your writings on flex nibs and it's clear you are focused on gold fountain pens. I have some experience with those, but much, much more with vintage, flexible steel dip pens.
I have a few responses.
1. you say "the shape of nibs and their tines is intricate" and this is even more the case for dip pens than for fountain pens, but even fountain pens have a wide range of shapes, including long and short tines, wide and narrow shoulders, etc... I'm assuming your thought is the shape is fairly irrelevant to the measurements you're making of flexibility. The shape is very important to attaining different levels of flexibility, as you point out later on, but initially the measurement should measure the same regardless of the shape.
2. Useful Line Width: I propose that useful line width is not a characteristic of a nib, but is instead a characteristic of ink, paper and feed if there is one. With steel dip pens there is no difference between useful line width and maximum line width. Because there is no feed, with the right ink and paper, you can make a line as wide as the tines can be separated. If we're talking about the characteristics of the nib, then this is not relevant, but if we're talking about writing with ink, on paper, with a nib on a pen, then it becomes a relevant, if adjustable, characteristic. Then it's more like maximum speed of a stock engine vs. maximum speed of an engine when you modify it.
3. In chapter 8, you talk about the technical means of determining a nib's characteristics: the spring constant and the preload. If I understand the difference correctly, one (the preload) is how much force is needed to begin the separation of the tines, and the spring constant is how much force is required to continue to spread the tines from that point? To me, while it would be extremely tiresome to measure, I suspect this line of spread vs. force would illustrate the most interesting variation across materials. With very flexible steel nibs, my anecdotal experience tells me that the pressure needed is not linear, but more of a curve. It starts out requiring greater force to overcome the friction keeping the tines together, then the force needed drops until you approach maximum feasible spread when it quickly rises. With less flexible nibs the line is more constant as there is a much smaller point of spreading. Now, this is based purely on my experience with steel, dip pens. I suspect gold responds differently. My much more limited experience with gold is that the force is more linear, but that may also be a function of the sharper tip on a steel pen, thus creating greater friction to begin the spread, than a round tipped gold nib.
4. In Chapter 11, bending, I can extrapolate from your diagram, that adding slits, piercings or cut-outs along the shoulders of a pen, you reduce the breadth, b, as well as reducing the resistance from the curved profile by effectively making the center of the nib more of a flat surface by separating it from the curved sides.
5. In Chapter 17A you show a dip nib with a complex piercing and cross grinding. (photo 9). First off, the grinding was done to steel nibs to reduce the thickness and increase flexibility. I'm not so sure of the actual effectiveness of these grinds, but they were considered important enough that after hand-grinding disappeared, it was replaced by machine-stamped grooves perpendicular across the nib in imitation of the grinding. There can sometimes be an additional straight grinding that is parallel to the axis of the nib, but those are more for decoration rather than function. (see my discussion of grinding and my description of how nibs were made to see the deliberate nature of the grind had nothing to do with de-burring) They do not have any impact on ink, as you speculate, because the grinding is on the top of the nib, while the underside, where the ink is held, is smoother. Gold dip nibs, on the other hand, are generally roughened on the underside to promote surface tension to keep the ink from sliding too quickly off the nib. (gold being "slicker" than steel) With a dip nib that has no feed, you are relying on surface tension to keep a pool of ink clinging to the surface of the underside of the nib, and the ink is drawn down the slit through capillary action from that pool of ink. The hole in the nib has very little impact on the amount of ink a nib is able to hold, and often the ink will gather on the back of the nib surrounding the hold but not bridging it.
6. In Chapter 17 B. - Wing Scallops, in your discussion you start to approach a complexity of terminology that is not talked about very much anymore: flexible pens vs. soft pens. With scallops cut out you add to the softness of the nib without appreciably impacting the flexibility of the nib. You can see some discussion of this here.
7. In Chapter 17D, towards the end, you show the Principality again. I think you are still under the impression that the hole in a dip pen actually has some kind of function. The only function holes have are to narrow the width of the nib at a point that may or may not impact flexibility. On this Principality, this narrowing has a great impact. On most standard pens, it has little to none. (I can show this by providing two pens with the same shape and hole placement but which have very different flexibilities)
The holes on steel dip pens cause a lot of confusion. Basically, they weren't really needed once makers started using less brittle spring steel, but were kept out of tradition, and because that's how they had always been made. On a steel pen you do not need the hole to prevent splitting, like you do on a gold pen. There is also no "breathing" done with the hole since there is no feed. As I mention above, the hole doesn't hold much, if any, ink on a properly prepped steel pen, and most capillary action takes place from the sides of the tines where the ink actually lies. The ink does not flow from the top down. If that was the case, a nib would hold very little ink. This is why wider nibs, like the Falcon or Spoon nibs can hold huge amounts of ink as they have wide sides.
There are whole classes of dip pens, usually made out of "pinchbeck," an allow of copper and zinc, that have no holes at all. They usually have very little flexibility (the material is not flexible) so the hole provides no advantage at all. And many old gold dip nibs have no hole either.
Overall a very interesting read. I suspect if you get yourself some good, vintage dip pens and examine them, I suspect you might learn a thing or two more about how to make truly effective flexible nibs. Just make sure you prep them correctly first, and you'll see what I mean about the capillary action and irrelevance of the hole.
Thanks for sharing