This is a great post. Emilio Sanchez-Vicario, who has an academy in FL, has said that the difference between US and Spanish (and you can extrapolate that to other countries) approach to teaching young players is that in the US they tend to start with the racquet. But in Spain, (and I think anyplace that emphasizes clay,) they start with the feet. At the very least, it explains why the US has done poorly at RG. Beyond that, it explains a lot, especially, as you say, how defense turns to offense.
I totally agree that even the greats work on their game, and "tweak" it. How did Roger come up with a new plan to defeat Rafa, so late career? He worked on taking his backhand earlier. That takes work, confidence, repetition, and I guess the bigger racquet head.
Toni Nadal said something a long time ago, and the quote is in Spanish, but the gist of it is that: "We could have decided to base our game on talent, but we decided to base it on hard work. Some days, your talent isn't enough, but if you can rely on your hard work, you might still win." This attitude gets used against Nadal, as if he's not talented, just a hard-worker. But there is a Zen in this. And a wise nugget. Great writers and artists tell us that they don't wait for the Muse...they go to work every day. So when she comes, they're prepared, and available. Think of the recent stat sheet littered with "talented" players that can't get the job done: Safin, Nalbandian, Dimitrov, Kyrgios (controversial, OK,) Rios. I'm tired of hearing that Federer and Djokovic have more natural talent than Nadal. They all work hard to be where they are, and I don't think, at this point, anyone can say one is especially more talented than the other. I think we all agree that Roger was the one with the most gifts from the outset. But even that had its own curse. Rafa and Novak, with something to chase, had to work harder. Then Roger had to adjust, to keep up.