N:
I read your essay on the floating weight/arm weight conundrum and agree (I think) in your conclusions.
As and observation -- not a criticism -- I think its too detailed and will lose the reader.
I think one are you could emphasize a bit more is what I consider to be critical.... IOW, get this wrong, and nothing will ever work.
And that -- along with posture -- is bench height, i.e. where the elbow is in relation to the keyboard.
For me, the perfect height is a bit higher than the level of the keyboard. This allows my forearm and hand to be at a very slight angle downward -- very slight -- which relieves most of the effort of elevating the forearm and hand to playing height, i.e. on the keys.
Once you get the posture right along with bench height -- elbow height to keyboard -- one has
the optimum set up to achieve the balance you're speaking of. At this point, it takes almost no effort to hold the forearm and hand at/on the keys, especially when the lateral movements of the arms are added in order to go from once place to another.
IMO, "weight" is really a misnomer and the old "weight school" has done more to physically either ruin or frustrate pianists to the point of quitting than any other single thing.
Ironically, and stubbornly, it is still taught -- i.e. the hand and fingers carry the weight of the arm in order to get a full rich sound -- which is not only the sure road to ruin, but hogwash from the get go!
As I mentioned yesterday, and as you know I am sure, the mechanical fact is the only thing that produces sound from the piano is the rapidity with which the hammer is catapulted into the string.
After the sound is made, all the weight in the world resting on the key bed yields nothing, and is an utter waste of energy.
I think a much better or at least more accurate visualization or metaphor, is that the forearm (and hand) act as "shock absorbers" for the energy the finger imparts into the key.
IOW a quicker more vigorous pulling down of the key (forte) requires a bit more shock absorption from a "floating leveraged stability" of the arm than a less vigorous key pull (piano).
This is achieved by adjusting the sense of light pressure on the top of the forearm behind the wrist about a third of the way toward the elbow, an adjustment which changes continually depending upon what's being executed by the fingers, position of the hand, and so on.
So everything is subject to a kind of floating leverage, and its not really dead weight or anything like it.
Of course all of this must be learned by experience and feel, and is really, really difficult to talk about in such a way as to avoid misunderstanding due to semantics, terminology, and so on.