As for as duplicating moves with Muga, I'm sure Yukimura and Sanada must have just tried duplicating any move they've seen. After all they've seen far more than anyone not in their school by definition (they're two time champs) so why wouldn't they just try everything they see in case there's a useful move? But they clearly turned Kirihara away from the Muga path, which implies they must know this path doesn't lead to anywhere useful and in terms of experience they're obviously the most qualified to say that copying people's moves doesn't get you anything really useful. I mean sure Sanada and Yukimura aren't the main characters so maybe Sanada and Yukimura uses Muga everywhere else we don't see them but there's just no reason to believe this is true especially given Yukimura pretty much thinks of Muga as a useless move.
Ryoma has pretty much nothing in common with second door but he can open it. If you talk about experience, both Yukimura and Sanada has far more of it than anyone else, again by the virtue of being on the champion team. If you talk about heritage, especially samurai heritage, Sanada obviously knows how to use a sword and Fu is a variant of Battou, and then there's the not-so-subtle fact that Sanada Yukimura is one of the greatest samurai ever. The third door cannot be opened by normal means but it didn't take anything super special to open the first two doors and Sanada and Yukimura are obviously really special to begin with. I mean Nioh can flat out clone the first two doors but he can't clone the Zeroth Serve in the match against Fuji, so in terms of difficulty it's harder to clone the Zeroth Serve than both of the first two doors!
Tezuka Zone isn't really a good move relative to the people we're talking about because it's said many times Tezuka Zone hurts your elbow in a serious way. While Tezuka Phantom exceeds what the human body can tolerate, don't forget Tezuka Zone is pretty much at the limit of what the human body can tolerate, so it's not something you want to do just for the sake of doing it.
Yet Tezuka, Yukimura, and Sanada are all guys who can return virtually any move in a 'normal' form, so to speak. Outside of Rai, I do not see any of those 3 characters encounter a move they cannot return normally. For a point of reference, Nioh while cloning Tezuka can return Fuji's 5th counter by hitting it back normally. At least within the same age group, almost any fancy technique is just a normal rally to those 3 guys, so why would any of them want to use a move that hurts yourself considerably to return a shot? Especially for Tezuka, who has a history of injury problems? Tezuka Zone is a move that'd be good for a power type character, like any of the Hado ballers. It's not a useful move for someone who can already return just about any special technique normally. In fact it seems like World of Ice was invented to justify why someone who can return just about any special technique normally would ever care about Tezuka Zone.
