It had the general outside shape of the WtD's wing, from what I spotted D: and the battle took precious minutes for me, I managed to get a good look at it.
No, it doesn't. I'm assuming you're talking about when he coats the Oblivion in Light during the final battle, right? All he does is turn its shape into that of a sword rather than a key. I just loaded it up and went into the fight to see it. Thankfully he stands around like an idiot for long periods of time. xD Easy to get a good pause screen with his sword in it. It narrows to a point with absolutely no discernible Way to the Dawn features to it whatsoever. I'm about 90% certain it's nothing more than a battle mechanic to make his fight harder. It's more just him using his powers over Light to extend the range and power of the Oblivion rather than randomly changing it into a completely new weapon.
I've been saying it for ages. And Xion "waking up" that second Keyblade inside of Roxas rather than just giving it to him supports it all the more. Sora and Roxas had half a Keyblade each due to them sharing Sora's power. Then Xion got her copy. Then Xion - with relations to Ven as we've seen - is defeated and another Keyblade is awoken inside Roxas. Once he fuses back into Sora, 1.5 + 0.5 Keyblades make for Sora to Duel Wield, only he needs help getting to that extra Keyblade, hence the arguable plot element of the Drive Forms - to help him reach Roxas and through him to Xion/Ven buried deep inside Roxas who's deep inside him.
Xion has connections to Roxas, thus to Ventus, it's not a direct connection... People look at her and see the person most closely associated with BOTH Xion and the viewer. Xigbar has not met Sora yet. He's not very close to Roxas. He's never met Namine or Kairi. Who else is left besides Ventus, who obviously connects to Roxas due to the spitting image similarity?
The half Keyblade idea makes some sense, I suppose, though I'm more under the impression that Roxas is using the Keyblade you're saying Xion awoke within Sora, the one related to Ventus, right from the beginning and that Xion, if anyone, shares the Keyblade with Sora. (run-on ftl) In this way, when Xion fuses back to Roxas, she returns Sora's actual Keyblade to Roxas, allowing him to combine Sora's and Ventus' together and dual wield. When he's fused back to Sora, same shiz, awakening the Ventus Keyblade in Sora.
As for the Drive Forms, it makes little sense, since Nomura has stated that Sora CAN dual wield in his base form, he just doesn't know he can or something along those lines. Perhaps that's talking of post-KH2 Sora, though, as it would make sense that he needs more help in order to draw upon Ventus' Keyblade, as it would rest within Roxas, not Sora, and Roxas is deep within Sora and has yet to accept that fact until very near the end of KH2 (which also happens to be when Sora's power seems to dramatically leap, being capable of splitting buildings in half with his Keyblade, for example).
actually, sora did. he's a heartless, and he uses his.
This is true, forgot about that. In that case, I'd say it makes a bunch of sense for Xehanort's Heartless to have awoken Riku's Keyblade in some capacity.