As to pulling with the middle finger, it is stronger than the index finger, pulls back straighter in the hand, and gets nerve inputs on both sides. Check out the digest of the paper by Walter J. Dorfner (long time lead FI for the VSP - now retired and deceased), on his development and experimentation with shooting that way.
It's a little off-topic but since you brought it up, I'll try to clear up a few things regarding hand anatomy.
I am sure Mr. Dorfner was a nice man, however he apparently knew little to nothing as to its anatomy and the bio-mechanics of how the hand works.
This is from your citation of his presentation:
THE ANATOMY OF THE FOREARM AND HAND
The anatomy of the forearm and the hand was then studied to determine how our muscles and tendons work with that method of shooting. Several interesting relationships were found.
1. The muscles and tendons used to flex the index finger are mechanically separated from those that flex the middle, ring, and little fingers. That allows the index finger to be flexed independently to pull a trigger.
The above statement and those that follow are fundamentally incorrect in virtually every aspect.
The muscles that flex all the fingers are located in the forearm, there are two such muscles to flex the 4 fingers (excluding the thumb for now)....
a "superficial" flexor that has 4 tendons arise from the end of the muscle belly and pass thru the carpal tunnel of the wrist, they then individually each pass thru their own tunnel of bone and tissue pulleys to guide them to their respective fingers, with each finger have 5 - 6 separate pulleys to control how they flex each joint of each finger. The "deep" flexor muscle also arises in the forearm with similar architecture of arising 4 tendons that also pass thru the carpal tunnel and accompany their respective "superficial" tendon to each finger. So, the muscle that flexes the small finger is the same muscle that flexes the other 3 fingers, both at the deep and superficial levels. Barring some anatomical variation, each finger normally flexes independently of its neighbor, at both the superficial and deep level of the tendons.
2. The muscles and tendons that are used to extend the index finger are isolated in the lower forearm. That allows the index finger to be extended and locked independently for pointing.
The extensor biomechanics of the fingers is a little more biomechanically complicated than the flexor side of the hand, however the extensor muscles of the fingers on on the hair-bearing side of the back of your forearm. Again each finger can normally be extended independently of each other....that's how we point with our index finger, flip the "bird" with your middle finger, etc...
3. When any finger is flexed, one tendon is used to move the tip of the finger, and another is used to move the middle part of the finger.
First, the tip of the finger is curled back by one tendon.
Then the middle part of the finger is flexed by another tendon. It pulls the middle part of the finger straight back, and the middle joint of the finger is used as the pivot for that action.
The independent action of the flexor tendons does not require sequential flexing on each finger......the deep tendon to each finger indeed flexes only the last joint of the finger....tested by holding a finger flat against a table and you will observe the last joint is capable of independent flexion.
The superficial tendon to each finger does flex the middle joint, its independent action is tested by isolating the finger flexed toward the palm while the other fingers are held back, you will see that the middle middle joint flexes however you cannot now flex the last joint of that finger.
4. The tendon in the middle finger that pulls the middle part of the finger towards the palm, also passes from the base of the middle finger through the center of the palm. That keeps the pulling force centered and straight back.
When the fingers are flexed into the palm, their tips all point to the same place on the palm......the directional pulling force for each finger is controlled by bone and tissue pulleys holding them individually in place, there is no directional mechanical advantage one finger has over the other.
5. The middle finger also can be flexed or extended individually.
All the fingers can be flexed or extended individually !
As for your comments on the hand,
"As to pulling with the middle finger, it is stronger than the index finger, pulls back straighter in the hand, and gets nerve inputs on both sides. "
they too are incorrect.......all the fingers have the same muscle (motor) powering their respective tendons, so one is not stronger per se.
As mentioned above, the middle finger is obviously in the "middle", however all the finger tips point to the same place, again with no advantage to any particular finger, limited only by the design of the trigger guard if present.
Finally, the middle finger is provided the same "nerve input" as the index finger when it comes to powering (motor nerve) them and their sensation to the skin of the fingers, both supplied by the median nerve which also accompanies the tendons on their passage thru the carpal tunnel.