First of all, we don't know if Yamato used GUYU before his match with Tezuka, nobody knew he used a special move until he explained it, so it's entirely possible that he had been using it for a while and just didn't tell anyone what it was.
Secondly, what exactly is stopping Inui from analysing the course of the real ball and swinging at the calculated point instead of the ball he's seeing? Data Tennis is, as the name suggests, based on data after all, so I don't think it's the same thing Tezuka does. I'm not saying it'll always work, but since he has Watefall as backup as well, just returning it sometimes should already be good enough.
Also, Ken, while I do agree with what you said about Marui's results in the other thread, isn't it true that Inui beat No. 17? On the other hand, Yamato is still only a 2nd stringer and if you want to believe the list we were given, only the 10th best at that.