That training, would it be more comparable to Kaidoh's boomerang snake, or to how Oishi and Kikumaru's synchro was mastered?
I don't recall Oishi and Kikumaru really practicing to master synchronizing whereas we've seen Ryoma actively stay on the court for some time, hitting through a basket of balls or two, so the former comparison seems more apt to me.
I agree on the metaphor part, though you could also write away the desert vs Zeus and the pirate skeleton battle as the interdimensional battle rather than a metaphor. At that point it just depends on how seriously you're willing to take Konomi, I guess ;p
Huh, associating the desert with interdimensional tennis never even crossed my mind. What gave you the impression that it is, aside from a player being drawn outside the tennis court? I always assumed it was to show just how hot it was when they were playing, but you're saying Zeus had something to do with creating the desert, correct?
As for the more general point though, if it's not a metaphor, I'm not sure how else to describe it. I hope everyone can agree that Tokugawa's not literally on a ship somewhere, fighting a skeleton pirate. It's clear that something different happens on the court, probably best illustrated when Tokugawa uses Black Hole for the first time (ch 121) since it briefly switches back and forth between the two scenes. And isn't that exactly what a metaphor is?
If the Glowing Shot is essentially channelling your feelings into the ball, would it be possible for the top-end middle schoolers(at the very least Atobe or Yukimura) to use it as well? Re: Atobe, he might be closer because of his Koori no Emperor serve, as it feels a bit like the Glowing Shot in a way.
I'm sure it's technically possible for them if they were put into the right situation. Ryoma and Akutsu can hit it, so there's no reason to assume the others can't in general. I kinda doubt we'll see either of them actually do it though.
As for Ice Emperor being similar to the glowing shot, I don't know. It seems to me that it's a more technical and "real" shot by exaggerating what happens when you hit, well, pretty much anything in general. Like, imagine the sound when you hit a normal serve with a nice and fast swing, it's much louder than when you use a slow swing. I think Atobe's serve is that idea turned up to eleven. Obviously not realistic by any stretch of the imagination, but more or less grounded in physics (similar to how the numbers Kawamura and Gin produce are
utterly ridiculous, but we wouldn't associate their shots with anything interdimensional).
The hypothesis about the Shot being fuelled by emotional/spiritual energy would be completely in line with the characters that can use it
(and Ryoga, Ryoma and Byou using it at will is also perfectly fitting, considering their (at least Ryoma's) attitudes considering tennis)
Plus, breaking their limits is something people can do in life-or-death situations (eg the mother that lifted a car her son was under until help arrived)
To finish up this post, here's the follow-up I promised: supposing that I've got the right idea when it comes to the glowing shot, I started wondering if we could use that to explain a second technique, namely Amadeus' Darkness. With the way Darkness specifically and Amadeus more generally are portrayed as counterparts to Byoudouin and the Glowing Shot, if GS means putting energy into the ball, isn't it possible that Darkness is the opposite, meaning Amadeus "removes" energy from the ball when using it?
Now what do I mean by that? Basically, if we say that GS is essentially hitting with heightened emotional/spiritual energy, Darkness would be hitting with no emotional weight attached to the shot. I also have a little more to back up that theory. Thinking back to the nationals, when Yushi overcame Momoshiro's predictions, there is a page right at the start where Momoshiro thinks he can't "see" Yushi anymore (ch 275). And how does Yushi achieve this effect? By sealing his heart, which can probably be closely associated with sealing his emotions. Likewise, when Amadeus uses Darkness for the first time, Akutsu says he can't see the ball itself anymore (only its shadow). Isn't it possible that the idea behind those abilities is similar too then?