It's been implied that he opened the door on DI. You can't do this unless you have a keyblade.
The theory= Ansem SoD sought Riku that night in the cave with the promise of seeing new worlds and getting off the islands. He introduced him to the keyblade and told him it would open the door, since he was the original chosen one. Which Riku complies, and summons the keyblade, or Ansem does it for him, so Riku could use it, even though he knows nothing about it that time. Only that it could open the door to get him out of there.
How would Ansem SoD have introduced him to the Keyblade? Ansem SoD could not summon a Keyblade. Riku could not summon a Keyblade at that time. As speculation goes, it's fine, but this theory doesn't correlate with anything we know about the story thus far. It's also directly contradictory: if Riku opens the Door to DI the night of the storm because Ansem SoD showed him how to do it, thereby eliminating the wall between worlds that kept outsiders from coming in, but Ansem SoD is a foreigner to the Islands, how did he get on the Islands before Riku opened the Door in order to show Riku how to open the Door?
This might be why later he asks Sora when they reunite "So this is called a 'keyblade'?
Alternatively, the line makes much more sense if it's interpreted as the first time he's seen a Keyblade and he's extremely curious about it. He knew the name of it because Maleficent had told him what it was, but he had never seen one with his own eyes. Otherwise, why would he be marveling over it ?
Why Zexion outright stated that he destroyed the islands and opened the door to darkness.
First of all, Zexion was trying to get under Riku's skin and force him to submit to the darkness inside him; he would have lied about anything to that end. He isn't a credible source. Also, the door to darkness does not equate to the Door to Destiny Islands in any way. In KH, Ansem SoD tells Riku he "showed no fear in stepping through the door to darkness"; the term is used pretty loosely to talk about utilizing the darkness in any way, especially when one is first introduced to it. (I know The Door to Darkness is also an actual door.)
Why he and Ansem SoD see each other on the destroyed islands, with a familiar smile and he creates a gateway for Riku in the Recocded flashback.
It's not clear what the significance of that scene was. Riku didn't appear to be phased by Ansem SoD when they ran into each other in Hollow Bastion and Ansem coaxed Riku into allowing him to take over his body. It was probably just establishing that they were already familiar by that time.
And you can't tell me no one else on the islands was interested in seeing the outside worlds.
First of all, that was exactly Riku's frustration with the world. Very few people were curious about anything beyond their own small community. Secondly, Kairi and Sora were interested in seeing outside worlds, too, although not to the extent that Riku was. You're also being a bit hyperbolic about this; maybe a number of other people did want to see the outside world, but without a Gummi Ship or another means of travel through the Lanes Between they wouldn't have been able to do so regardless of the walls between worlds. There's nothing in this argument to prove the walls were in place.
Again, Riku and Xehanort could and did because they are keyblade wielders.
This may be true for Xehanort, but it doesn't coincide with what we know about Riku. Perhaps Nomura plans to retcon this into the story as well, but for now, there isn't a great deal of evidence to support it.
