Man reading your entry on the keyblade graveyard just makes me mad, not at you but at the game. The keyblade graveyard was such a disappointing final world and every KH game before and after BBS, even Coded, knew how to make a good final world. You know in the multiplayer there are arenas based on the different worlds one is based on the keyblade graveyard and that single room is far more engaging than the entire KG was as a whole. I really wish we could have gotten more than 3 rooms of flat waste land, just do it like EotW in KH1 that was a wasteland world but still with all kinds of cool platforming. I will be forever scarred by the wasted potential of this world and it's doubly wasted because it actually damages the story a bit when you consider TAV come from separate directions.
Aqua's line to Terra makes sense she is basically rightfully trying to get him to think about the consequences of his actions and more importantly that ISN'T a hypothetical. At this point to her it's not a matter of "if" Terra will go astray it's only a matter of "when" as she has completely lost faith in him. She is basically saying "You killed our master and you will go astray when you fight Xehanort, tell me Terra what reason do you have to believe things will be different, what reason do you have to believe you're not just going to defile Eraqus's memory."
When he says keyblades of light and darkness he's talking about the hearts aligned with them, not keyblades from the realm of darkness but keyblades being used by people whose hearts had been consumed by darkness or more specifically things like greed and lust. I also want to say it is really annoying this is literally the only time in the entire game Braig and Ven see one another, Days built it up to be so much more meaningful. Likewise the battle the follows is part of the reason I dislike Ventus, all he had to do was defend Aqua not actually fight Vanitas. I think at this point was when I realized Ventus has done nearly nothing all game but cause problems for his friends and make things worse, and I really don't think that single gesture at the end he does makes up for it by any stretch of the imagination.
Oh and you're sorta incorrect on the face thing. Vanitas has Sora's face it's not the other way around. In Nomura's own words when Sora connected to Ventus where Vanitas was still connected to that affected Vanitas making him look like Sora. Sora wasn't affect it was Vanitas who was affected.
How did you decide on the design for their faces?
Well Terra’s look was already a decided thing, we just had to make him look a bit younger. I knew that Ventus should look either like Sora or Roxas, and I wasn’t sure which one to go with, but I thought Vanitas looking like Sora would have a bigger impact so I had Ventus look like Roxas instead. And there is a reason that Vanitas looks like Sora. As Sora filled in Ventus’s fractured heart, the fractured part (Vanitas) was affected by Sora and ended up with Sora’s face. So if it had been Riku who had filled in Ventus’s heart, Vanitas would have looked like Riku.
I mean the other way would make more sense but sadly that is not the case, Vanitas got Sora's face aged up to fit his current body's age which was probably around 15. Also I never got the sense Vanitas was losing, from the sound of his dialogue in the transition it sounds like he was just toying with Ventus and then got bored and was ready to just fully absorb him. Represented by him smashing the heart station and getting ready to go nuts on him with his powers. What he didn't account for was that Aqua had given Ventus a good luck charm that lets him borrow the powers of people who connect to his heart even if that connection is forceful. In fact I'll give BBS credit to this, since they intended for Terra to be the first story it's nice that they foreshadowed in his story that you can get D-links from people forcibly connecting to your heart. I thought that was a clever/under appreciated bit of foreshadowing likewise the D-link thing with Ventus was a really great way to answer that question Ventus posed to Aqua at the start of the game "So this isn't a real good luck charm? Well that remains to be seen but I did work a little magic on it".