The funny thing is that Yen Sid never teaches Sora and Riku the "proper" way of the keyblade, they are learning it on their own once again in KH3D.
Honestly introducing another time travel method that is as convoluted as Xehanort's was one of the worse of KH3D's bag of bad ideas.
Well, can't say I don't agree time travel was a bad idea here... although I'm pretty convinced that they were being guided by the sleeping worlds to relearn - they weren't totally independent like in kh1 and kh2. That goes along with how their abilities transferred into the real world after unlocking the seven sleeping keyholes.
Sending Sora and Riku back in time to gain access to the Sleeping Worlds is a strange idea, it becomes even more strange when they awake in TWTNW in the present.
Well, it does make sense, really. They needed to go with Destiny Islands as it fell into sleep in order to gain access to the others. In kh1, Sora and Riku went through dark portals (kinda like the Organization's) which is why they didn't enter the sleeping worlds. As for going to the present, I think the sleeping worlds were kind of exempt from time, and only after the Organization led them back into reality did they enter the present.
The seven keyholes of sleep is another problem. Why are they so important? Why are there more than seven in the game? Is there at least one for every world and did every world wake up (with the expection of the fake Traverse Town) after they were all unlocked, even the unshown one's like Dumbo's homeworld?
I'm assuming unlocking these keyholes both freed them from sleep and granted Sora and Riku new powers. Now, where diid you see more than seven? The only one I can recall is Traverse Town, and I believe that the first time didn't fully unlock it.
If you're talking about Riku having to unlock the keyholes, I think they required two keys/hearts to complete it, kinda like how the Door to Darkness in kh1 had the same requirements (KH3D does draw a lot of parallels to kh1, from what I've seen). Also, Riku is in Sora's dreams, so that could be another explanation. Either he was unlocking them as figments of Sora's dreams, or his unlocking of them transcended Sora's dreams and completed the 2nd half.
If Master Xehanort and Young Xehanort can be part of the True Organization XIII why can't he just summon more versions of himself to fill all 13 spots?
If Merlin and Yen Sid have more powerful magic than Xehanort then why is he a problem at all? Can't they sent Sora or someone else to solve this problem in the past (and/or save TVA for that matter).
I don't think Young Xehanort was a part of the true Org XIII. After all, he had to be snapped back to his own time, so that rules out all past versions of himself, really (including Xemnas, Ansem, etc.).
Now, Timeless River actually set a precedent for time travel in Kingdom Hearts - you can't change events in the past. Well, you CAN, but anyone who knows the magical version of time travel probably follows some code that forbids them from doing so. That's why Merlin told Sora not to "give in to dark forces" - Yen Sid was following this same code, so Sora and Riku couldn't interfere with the past. Lord knows the consequences.
I have not lost hope that the future games will eventually overcome this, but at the moment I have serious doubts giving that Nomura doesn't seem to be sure about the rules of his own universe, even if they were laid out in some detail and only expanded very carefully in the past.
Aww, I have faith in Nomura! KH3D only really TOUCHED on a lot of delicate stuff. That's a big reason why I'm looking forward to KH3. It's gonna clear up any confusion on whatever KH3D started.