I don't buy it; holding the fist "hollow" does not confer any particular advantage: Yang changed it to accommodate his upscale students; it's the same reasoning behind his taking out the seminal movement (from a Taoist alchemical perspective) of Arhat Grinding the Mortar - stomping on the ground would be unseemly; so too went the double jump front kick in the middle of the Second Chapter, the low spinning sweep (Iron Broom) at the end of it and the double jumping / spinning kick at the end of the form (if it's called Double / Twin Lotus, why is there only one kick in the extant form?); these are all movements in the version of the "pre-Beijing" (as my teacher refers to it) Yang form I learned, and there is no "hollow fist" anywhere to be found; at the same time, we do not hold the closed fist "tightly clenched" either, as that does create unnecessary tension during delivery as described above (basically, firing finger flexors maximally has a synergistic effect up through flexori carpi and biceps, which certainly slows the extension of the arm - so no need to talk about "energy" abstractly, it's very well-describable from a neuromuscular perspective) but on impact the fist is tightened; but this is not different from what I have seen in other styles, internal or otherwise: even my old TKD instructor used to punch this way; and William would understand this as well, what with his boxing background...
this I agree with completely; so-called "reverse" breathing creates a more stable base of support for the second phase of diaphragmatic contraction, because it prevents the abdominal viscera from descending as much as they would during "belly" breathing; so with the central tendon of the diaphragm stabilized, the peripheral fibers are able to contract more fully / strongly to elevate / externally rotate the ribs, allowing for more air to enter the lungs; the contributes to, among other things, better spinal health, better autonomic tone, better lymphatic drainage and better abdominal visceral function; this is all because of the "compression" (followed by a "decompression" part) that creates a "backflow" effect, pushing lymph and venus blood out of the organ systems and back into the vena cava;
true
I am a big fan of the 4# ball, use it in PT w/my kids all the time - great for getting them to coordinate breathing, movement from the core and visual-vestibular integration; the basic position is to have them hold the ball at heart level with what basically amounts to a Lotus Mudra (but don't tell them that!) and go from there; incidentally, I also do push-hands drills with some of 'em, it is a pretty efficient means by which to identify and treat deficits in proprioception, bilateral integration / coordination, etc., and gives me the opportunity to "model" via tactile input what I want them to be doing...