Good Discussion, late response.
I agree with Loeffler and Six Rabbit,
I shoot a short strong California style bow. I use button brush (willow) and elderberry for my arrows. The type of plant species determines spine. For instance I only like button brush, and stay away from all other willows as I find them weak in spine. I like to find shafts that are closest to the diameter I want, so I'm not sanding all day to get the desired diameter. When all this is completed, I will always get arrows with exact spine. Over the years I honed in on the appropriate dimensions. Now, my eye picks out shoots instinctively. Sometimes I spot shoots along the road as I drive. No doubt natives developed this skill early with their fathers teaching them.
When I started I would hastily pick 20 shoots I thought would be great. I would usually only get 10 -12 good ones. Now I pick five shoots with a careful eye and they will all reach arrow status. Every once in a while I find some bug damage.
Elderberry is a bit more troublesome, but if you find the right one - its great. I've noticed elderberry that's next to a good water source (spring) grows quickly with thin walls - no good. Bushes I find away from water sources develop thick walls. I frequent these same bushes over and over. When you find an elderberry with thick walls, but with overall narrow diameter, it outshoots any other arrow material I have used.
I also keep a few "perfect arrows" aside as reminders of the dimensions I like.
The stiffer and more narrow the better, as far as short powerful bows go... my opinion.
Don't doubt natives had tricks, skills, and knowledge we just don't know of. I've heard for some plants picking shoots at night in between moon phases produces the best shoots.
Ishi knew the results of this hazel shoots from making arrows his whole life. I would expect most of us would develop that instinctively over time and wouldn't have to measure, weigh, etc our arrows.
I don't know a darn thing about spining arrows, but I assure you my 25'' arrows fly real nice.
Also, as for the archers paradox, it plays no role in the style of shooting that Ishi had (and other California tribes). I only shoot like Ishi, and the arrow never touches the bow or my grip hand because of the twisting motion during the release which gives room for the arrow to pass. Again my opinion, but the paradox seems to be with European/American style shooting.