Aqua is trapped in the RoD during the events of I and II... sooooo you, sir, are wrong.
Don't write off what he said as wrong automatically. He said Riku
may have gotten some form of training from Aqua while wandering in and out of the Realm of Darkness. It's not being stated as fact. Riku
could have run into Aqua, and Aqua
could have helped propel Riku to eventually getting his Souleater to evolve to Way of the Dawn. We don't know. It's all speculation that will hopefully be answered in BBS v2.
It's just like me thinking that
perhaps, Riku had a run-in with the Lingering Sentiment in the Keyblade Graveyard sometime in KH2 while SDG were world hopping their second time around, and something there helped him propel Riku's heart to get strong enough to have his Souleater evolve to Way of the Dawn. This of course would mean that Riku
might already be aware of Terra's existence, and his connection to him in some way, shape, or form. But once again, this is speculation.
That being said:
1) I feel Aqua and Terra were in no means ready to become masters. But it's not their fault. They were being taught by a light zealot who didn't understand the meaning of balance.
2)I think Sora and Riku's path could go one of two ways: Both of them become masters, or only Riku becomes master. Because despite their much smaller experiences with the keyblade, Riku's learned very important things that both Terra and Aqua have failed to grasp (especially Aqua). That to me is worth more than years and years of training. And because of that, I feel that Riku is slightly more qualified to become a keyblade master than even Sora. Yeah Sora's got ridiculous amounts of raw, untapped potential, but he's never really confronted his own darkness yet, and he does at least have small amounts of it. He may readily accept Riku despite his brush with darkness, but Sora still has to learn about balancing light and dark himself.
