Well, there are a few things to note here
1. You say Sora wields the KK and oathkeeper, but, if we compare it to KH1, all the previews were with Sora using the KK, the DEFAULT WEAPON, I would assume that since whenever he goes into Brave form in the KH2 trialers he has the default weapon (KK), that that'd be what he uses when he fuses, so if it showed him with a different keyblade then with the KK and oathkeeper after fusing I'd say your right, but I'd assume its: Currently Wielded Keyblade+ Oathkeeper, also however, the DW throws his keyblade to Riku then draws another, thats two seperate keyblades, not the same one (and Sora doesnt have 3 [To our knowledge]).
2. Thats Riku saying "Wheres Sora" in his head, not the DW, (I explained this in an earlier post)
3. The DD scene lasts too long to be fusion (recall that he walked down the street to the location of the battle, and we saw no Goofy to fuse with) , and when Sora uses it in the trialers he glows red with an electrical aura, this is not observed in DD.
4. If he fused with Goofy, where is Donald?
And thats all my points for today, thank you for your time