I realize there might be a matter of semantics, or there is something I am missing, but to me there is a huge difference between Roy's proposed idea of finger follow through as if plucking the key from the surface and then the finger still keeps moving, following through vs. H. M. and A.T. economical motion of the firing finger precisely to 10.5 mm down and after that immediately releasing the energy once the finger reaches the key bed.
Ok. I think that this degree of plucking business is for Roy's demonstration purposes... at least that's my interpretation.
I do NOT agree that the "norm" in playing requires this, and in fact will produce inflamation eventually.
Glenn Gould -- who had a very "technique" suffered from boughts of tendonitits, if my recollection serves me, as did Horowitz, who also used this technique at times -- both of these legends also playing in a rather "tense" manner.
Roy's main contribution, IMO, is to show how light and floating the arm is, that the fingers play from on the key -- not hitting it -- and that the arm' main function is to position the hand and fingers.
We see exactly this with Art and Michellangeli.
As for key travel, I agree with you that it is tiny, and even from a somewhat "already depressed state", IOW, not only on the key but somewhat "in" the key. And similar to what you say, once the required effort has been expended to "fire" the hammer into the string via the key lever, any further energy from the finger is a waste.
To go a bit further with this key travel business, its even hard to say how far down the finger goes, since depending on where you play the key, the distance down is greater or smaller.
For instance, if you play a white key at the edge, this is the greatest distance the key travels down vertically. If you play it as close to the fall board as possible, then this is the least distance it travels down (since its a lever and closer to the fulcrum, I think).
I bring this up, because to attempt to describe a technique -- outside of important and undeniable basics -- is impossible, since each cluster of notes to be played has different "3 dimensionality"
dependint on the pianist, with different size and shape hands, and other piano playing human "equipment", and even depending on how the pianist does phrasing, etc.
So I think that after a certain basic point, further discussion gets more and more complex, and may well take one further from the goal of understanding than closer.
I think that if I were to vote for the very best examples of "how to play" from the pianists we've discussed, I have to conclude that Michellangeli and Art Tatum demonstrate everything one needs to know.
Tatum reminds us that music should be fun and relaxing, not just intellectual and "high art" -- a point of great importance, IMHO, that often gets lost in just following the "classicists".