I disgaree Reyes sound is good enough. It is not bad, it is ok, but there are others by far better. I say this from what I hear in the videos you put here. But the quality of them are poor, youtube is not a good way to check this, so it is difficult to make a point, and I agree a may be wrong or simply my taste may be different.
On the physics and mass, I didn't read the book, but the physics you explained quickly here is not correct. No matter which joints you use, the mass is from the whole hand and forearm, depends more on the weight you leave at the point of the guitar you touch with arm and the weight you let it go toward the strings. If you let arm/wrist loose and just pluck the strings, the hand bounces back upwards, as a reaction. So the whole hand/arm mass is going down there when you hold hand down. The weight difference from middle finger and the other fingers are very little compared to hand weight and arm weight you can put over them. The point of the guitar contact and arm weight you put over the fingers is by far the more important mass in the system, so it doesnt matter which finger you use.
I agree sometimes middle finger seems to have louder sound, it happens a lot to students in regular technique, but physically it seems more obvious that the reason is because it is longer than others which helps going deeper down, I can see the person digging more with this finger, it is not related to its mass quantity. Hand poistining is not an easy task, when you get better to one finger, it goes worse to the other.
Another point, is that the string energy of sound is from its release, you are not kicking it, it doesn't move further down when you release it. Use of arm weight or whatever is to make it easier for the player to displace the string, but no matter how much mass you use, the string vibrates from the realese, so it is from its own elasticity and weight, and from the displacement you've given to it, it doesn't matter how much mass you used to displace it. The easier way of displacing further is leting the whole arm mass go there, rather than a simple finger.
Certainly the way you release, the way you let it speed up over the finger, the nail etc. also make a big difference in final sound result, it is not only a matter of simple displacement. It is certainly not an easy task to describe the physics in detail. This last aspects where it seams to me diffcult to get a nice sound with the backwards pluck. I think flesh from regular pluck helps avoiding noise from direct nail contact, and also the nails is not symmetrical, so if you file it well for a regular pluck it won't be as good for the backward one and vice-versa.